diff --git a/app/helpers/system_note_helper.rb b/app/helpers/system_note_helper.rb
index 209bd56b78ac0ad2d82a54b4bb4090d98aefbebc..08fd97cd0481566dee8f38ea8167abe02127c124 100644
--- a/app/helpers/system_note_helper.rb
+++ b/app/helpers/system_note_helper.rb
@@ -18,7 +18,8 @@ module SystemNoteHelper
     'milestone' => 'icon_clock_o',
     'discussion' => 'icon_comment_o',
     'moved' => 'icon_arrow_circle_o_right',
-    'outdated' => 'icon_edit'
+    'outdated' => 'icon_edit',
+    'duplicate' => 'icon_clone'
   }.freeze
 
   def icon_for_system_note(note)
diff --git a/app/models/system_note_metadata.rb b/app/models/system_note_metadata.rb
index 414c95f770583f55df66f1fc0fa265ed2ff8b307..0b33e45473b719fcec2bb4cb8df61787bc67d600 100644
--- a/app/models/system_note_metadata.rb
+++ b/app/models/system_note_metadata.rb
@@ -1,7 +1,8 @@
 class SystemNoteMetadata < ActiveRecord::Base
   ICON_TYPES = %w[
     commit description merge confidential visible label assignee cross_reference
-    title time_tracking branch milestone discussion task moved opened closed merged
+    title time_tracking branch milestone discussion task moved
+    opened closed merged duplicate
     outdated
   ].freeze
 
diff --git a/app/services/issuable_base_service.rb b/app/services/issuable_base_service.rb
index 9078b1f0983c9dd2d4f1047ef0be576cd6a1e8e3..ea4977291153c9668c8e5161b17aa9a8197e2307 100644
--- a/app/services/issuable_base_service.rb
+++ b/app/services/issuable_base_service.rb
@@ -58,6 +58,7 @@ class IssuableBaseService < BaseService
       params.delete(:assignee_ids)
       params.delete(:assignee_id)
       params.delete(:due_date)
+      params.delete(:canonical_issue_id)
     end
 
     filter_assignee(issuable)
diff --git a/app/services/issues/base_service.rb b/app/services/issues/base_service.rb
index 34199eb5d1373347188ad664d01a17eb84eaa29c..4c198fc96eaa7205bff45ea4f7d7dbd077310540 100644
--- a/app/services/issues/base_service.rb
+++ b/app/services/issues/base_service.rb
@@ -7,6 +7,14 @@ module Issues
       issue_data
     end
 
+    def reopen_service
+      Issues::ReopenService
+    end
+
+    def close_service
+      Issues::CloseService
+    end
+
     private
 
     def create_assignee_note(issue, old_assignees)
diff --git a/app/services/issues/duplicate_service.rb b/app/services/issues/duplicate_service.rb
new file mode 100644
index 0000000000000000000000000000000000000000..5c0854e664d219790e4a1495fc1e61fb7d73b282
--- /dev/null
+++ b/app/services/issues/duplicate_service.rb
@@ -0,0 +1,24 @@
+module Issues
+  class DuplicateService < Issues::BaseService
+    def execute(duplicate_issue, canonical_issue)
+      return if canonical_issue == duplicate_issue
+      return unless can?(current_user, :update_issue, duplicate_issue)
+      return unless can?(current_user, :create_note, canonical_issue)
+
+      create_issue_duplicate_note(duplicate_issue, canonical_issue)
+      create_issue_canonical_note(canonical_issue, duplicate_issue)
+
+      close_service.new(project, current_user, {}).execute(duplicate_issue)
+    end
+
+    private
+
+    def create_issue_duplicate_note(duplicate_issue, canonical_issue)
+      SystemNoteService.mark_duplicate_issue(duplicate_issue, duplicate_issue.project, current_user, canonical_issue)
+    end
+
+    def create_issue_canonical_note(canonical_issue, duplicate_issue)
+      SystemNoteService.mark_canonical_issue_of_duplicate(canonical_issue, canonical_issue.project, current_user, duplicate_issue)
+    end
+  end
+end
diff --git a/app/services/issues/update_service.rb b/app/services/issues/update_service.rb
index cd9f9a4a16e02724961b0d6384e4733336650d1c..8d918ccc635ec2ebb286575a7b39cd1e9217068e 100644
--- a/app/services/issues/update_service.rb
+++ b/app/services/issues/update_service.rb
@@ -5,6 +5,7 @@ module Issues
     def execute(issue)
       handle_move_between_iids(issue)
       filter_spam_check_params
+      change_issue_duplicate(issue)
       update(issue)
     end
 
@@ -53,14 +54,6 @@ module Issues
       end
     end
 
-    def reopen_service
-      Issues::ReopenService
-    end
-
-    def close_service
-      Issues::CloseService
-    end
-
     def handle_move_between_iids(issue)
       return unless params[:move_between_iids]
 
@@ -72,6 +65,15 @@ module Issues
       issue.move_between(issue_before, issue_after)
     end
 
+    def change_issue_duplicate(issue)
+      canonical_issue_id = params.delete(:canonical_issue_id)
+      canonical_issue = IssuesFinder.new(current_user).find_by(id: canonical_issue_id)
+
+      if canonical_issue
+        Issues::DuplicateService.new(project, current_user).execute(issue, canonical_issue)
+      end
+    end
+
     private
 
     def get_issue_if_allowed(project, iid)
diff --git a/app/services/quick_actions/interpret_service.rb b/app/services/quick_actions/interpret_service.rb
index 6f82159e6c7c0df23fee2be24e01131dc5d3b37a..5dc1b91d2c0486a67b682e113e691604bdb35528 100644
--- a/app/services/quick_actions/interpret_service.rb
+++ b/app/services/quick_actions/interpret_service.rb
@@ -471,6 +471,24 @@ module QuickActions
       end
     end
 
+    desc 'Mark this issue as a duplicate of another issue'
+    explanation do |duplicate_reference|
+      "Marks this issue as a duplicate of #{duplicate_reference}."
+    end
+    params '#issue'
+    condition do
+      issuable.is_a?(Issue) &&
+        issuable.persisted? &&
+        current_user.can?(:"update_#{issuable.to_ability_name}", issuable)
+    end
+    command :duplicate do |duplicate_param|
+      canonical_issue = extract_references(duplicate_param, :issue).first
+
+      if canonical_issue.present?
+        @updates[:canonical_issue_id] = canonical_issue.id
+      end
+    end
+
     def extract_users(params)
       return [] if params.nil?
 
diff --git a/app/services/system_note_service.rb b/app/services/system_note_service.rb
index da0f21d449ab6909bbb94118a8aa6aa0c6f7410b..2dbee9c246e270a9e43913bae312465bdcde96a5 100644
--- a/app/services/system_note_service.rb
+++ b/app/services/system_note_service.rb
@@ -552,6 +552,44 @@ module SystemNoteService
     create_note(NoteSummary.new(noteable, project, author, body, action: 'moved'))
   end
 
+  # Called when a Noteable has been marked as a duplicate of another Issue
+  #
+  # noteable        - Noteable object
+  # project         - Project owning noteable
+  # author          - User performing the change
+  # canonical_issue - Issue that this is a duplicate of
+  #
+  # Example Note text:
+  #
+  #   "marked this issue as a duplicate of #1234"
+  #
+  #   "marked this issue as a duplicate of other_project#5678"
+  #
+  # Returns the created Note object
+  def mark_duplicate_issue(noteable, project, author, canonical_issue)
+    body = "marked this issue as a duplicate of #{canonical_issue.to_reference(project)}"
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
+  end
+
+  # Called when a Noteable has been marked as the canonical Issue of a duplicate
+  #
+  # noteable        - Noteable object
+  # project         - Project owning noteable
+  # author          - User performing the change
+  # duplicate_issue - Issue that was a duplicate of this
+  #
+  # Example Note text:
+  #
+  #   "marked #1234 as a duplicate of this issue"
+  #
+  #   "marked other_project#5678 as a duplicate of this issue"
+  #
+  # Returns the created Note object
+  def mark_canonical_issue_of_duplicate(noteable, project, author, duplicate_issue)
+    body = "marked #{duplicate_issue.to_reference(project)} as a duplicate of this issue"
+    create_note(NoteSummary.new(noteable, project, author, body, action: 'duplicate'))
+  end
+
   private
 
   def notes_for_mentioner(mentioner, noteable, notes)
diff --git a/app/views/shared/icons/_icon_clone.svg b/app/views/shared/icons/_icon_clone.svg
new file mode 100644
index 0000000000000000000000000000000000000000..ccc897aa98f834d65d0865639a98bbb6b6ecc058
--- /dev/null
+++ b/app/views/shared/icons/_icon_clone.svg
@@ -0,0 +1,3 @@
+<svg version="1.1" xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink" width="14" height="14" viewBox="0 0 14 14">
+<path d="M13 12.75v-8.5q0-0.102-0.074-0.176t-0.176-0.074h-8.5q-0.102 0-0.176 0.074t-0.074 0.176v8.5q0 0.102 0.074 0.176t0.176 0.074h8.5q0.102 0 0.176-0.074t0.074-0.176zM14 4.25v8.5q0 0.516-0.367 0.883t-0.883 0.367h-8.5q-0.516 0-0.883-0.367t-0.367-0.883v-8.5q0-0.516 0.367-0.883t0.883-0.367h8.5q0.516 0 0.883 0.367t0.367 0.883zM11 1.25v1.25h-1v-1.25q0-0.102-0.074-0.176t-0.176-0.074h-8.5q-0.102 0-0.176 0.074t-0.074 0.176v8.5q0 0.102 0.074 0.176t0.176 0.074h1.25v1h-1.25q-0.516 0-0.883-0.367t-0.367-0.883v-8.5q0-0.516 0.367-0.883t0.883-0.367h8.5q0.516 0 0.883 0.367t0.367 0.883z"></path>
+</svg>
diff --git a/changelogs/unreleased/26372-duplicate-issue-slash-command.yml b/changelogs/unreleased/26372-duplicate-issue-slash-command.yml
new file mode 100644
index 0000000000000000000000000000000000000000..3108344e0bf7a3311c7db3d7582a959eb84cffe0
--- /dev/null
+++ b/changelogs/unreleased/26372-duplicate-issue-slash-command.yml
@@ -0,0 +1,4 @@
+---
+title: Added /duplicate quick action to close a duplicate issue
+merge_request: 12845
+author: Ryan Scott
diff --git a/doc/user/project/quick_actions.md b/doc/user/project/quick_actions.md
index 19b51c83222df56a9f3fcf12ada2ef5ddef799e4..ce4dd4e99d54b7c57dc72f089b575a1f6696de2e 100644
--- a/doc/user/project/quick_actions.md
+++ b/doc/user/project/quick_actions.md
@@ -37,3 +37,4 @@ do.
 | `/target_branch <Branch Name>` | Set target branch for current merge request |
 | `/award :emoji:`  | Toggle award for :emoji: |
 | `/board_move ~column`      | Move issue to column on the board |
+| `/duplicate #issue`        | Closes this issue and marks it as a duplicate of another issue |
diff --git a/spec/features/issues/user_uses_slash_commands_spec.rb b/spec/features/issues/user_uses_slash_commands_spec.rb
index 4740402dc0182cbb6387fb2bc60d1803bad84e11..0c3c27e3e45d801e8489333c25df2f17e4b8fc9c 100644
--- a/spec/features/issues/user_uses_slash_commands_spec.rb
+++ b/spec/features/issues/user_uses_slash_commands_spec.rb
@@ -118,5 +118,42 @@ feature 'Issues > User uses quick actions', feature: true, js: true do
         expect(page).not_to have_content '/wip'
       end
     end
+
+    describe 'mark issue as duplicate' do
+      let(:issue) { create(:issue, project: project) }
+      let(:original_issue) { create(:issue, project: project) }
+
+      context 'when the current user can update issues' do
+        it 'does not create a note, and marks the issue as a duplicate' do
+          write_note("/duplicate ##{original_issue.to_reference}")
+
+          expect(page).not_to have_content "/duplicate #{original_issue.to_reference}"
+          expect(page).to have_content 'Commands applied'
+          expect(page).to have_content "marked this issue as a duplicate of #{original_issue.to_reference}"
+
+          expect(issue.reload).to be_closed
+        end
+      end
+
+      context 'when the current user cannot update the issue' do
+        let(:guest) { create(:user) }
+        before do
+          project.team << [guest, :guest]
+          gitlab_sign_out
+          sign_in(guest)
+          visit project_issue_path(project, issue)
+        end
+
+        it 'does not create a note, and does not mark the issue as a duplicate' do
+          write_note("/duplicate ##{original_issue.to_reference}")
+
+          expect(page).to have_content "/duplicate ##{original_issue.to_reference}"
+          expect(page).not_to have_content 'Commands applied'
+          expect(page).not_to have_content "marked this issue as a duplicate of #{original_issue.to_reference}"
+
+          expect(issue.reload).to be_open
+        end
+      end
+    end
   end
 end
diff --git a/spec/services/issues/duplicate_service_spec.rb b/spec/services/issues/duplicate_service_spec.rb
new file mode 100644
index 0000000000000000000000000000000000000000..82daf53b1737110165be376f43033c8873b4e7e9
--- /dev/null
+++ b/spec/services/issues/duplicate_service_spec.rb
@@ -0,0 +1,80 @@
+require 'spec_helper'
+
+describe Issues::DuplicateService, services: true do
+  let(:user) { create(:user) }
+  let(:canonical_project) { create(:empty_project) }
+  let(:duplicate_project) { create(:empty_project) }
+
+  let(:canonical_issue) { create(:issue, project: canonical_project) }
+  let(:duplicate_issue) { create(:issue, project: duplicate_project) }
+
+  subject { described_class.new(duplicate_project, user, {}) }
+
+  describe '#execute' do
+    context 'when the issues passed are the same' do
+      it 'does nothing' do
+        expect(subject).not_to receive(:close_service)
+        expect(SystemNoteService).not_to receive(:mark_duplicate_issue)
+        expect(SystemNoteService).not_to receive(:mark_canonical_issue_of_duplicate)
+
+        subject.execute(duplicate_issue, duplicate_issue)
+      end
+    end
+
+    context 'when the user cannot update the duplicate issue' do
+      before do
+        canonical_project.add_reporter(user)
+      end
+
+      it 'does nothing' do
+        expect(subject).not_to receive(:close_service)
+        expect(SystemNoteService).not_to receive(:mark_duplicate_issue)
+        expect(SystemNoteService).not_to receive(:mark_canonical_issue_of_duplicate)
+
+        subject.execute(duplicate_issue, canonical_issue)
+      end
+    end
+
+    context 'when the user cannot comment on the canonical issue' do
+      before do
+        duplicate_project.add_reporter(user)
+      end
+
+      it 'does nothing' do
+        expect(subject).not_to receive(:close_service)
+        expect(SystemNoteService).not_to receive(:mark_duplicate_issue)
+        expect(SystemNoteService).not_to receive(:mark_canonical_issue_of_duplicate)
+
+        subject.execute(duplicate_issue, canonical_issue)
+      end
+    end
+
+    context 'when the user can mark the issue as a duplicate' do
+      before do
+        canonical_project.add_reporter(user)
+        duplicate_project.add_reporter(user)
+      end
+
+      it 'closes the duplicate issue' do
+        subject.execute(duplicate_issue, canonical_issue)
+
+        expect(duplicate_issue.reload).to be_closed
+        expect(canonical_issue.reload).to be_open
+      end
+
+      it 'adds a system note to the duplicate issue' do
+        expect(SystemNoteService)
+          .to receive(:mark_duplicate_issue).with(duplicate_issue, duplicate_project, user, canonical_issue)
+
+        subject.execute(duplicate_issue, canonical_issue)
+      end
+
+      it 'adds a system note to the canonical issue' do
+        expect(SystemNoteService)
+          .to receive(:mark_canonical_issue_of_duplicate).with(canonical_issue, canonical_project, user, duplicate_issue)
+
+        subject.execute(duplicate_issue, canonical_issue)
+      end
+    end
+  end
+end
diff --git a/spec/services/issues/update_service_spec.rb b/spec/services/issues/update_service_spec.rb
index d0b991f19abbc9e3b27750eadff3a5e44e2b5548..064be940a1c889046436179827a3bff0b6b7f88c 100644
--- a/spec/services/issues/update_service_spec.rb
+++ b/spec/services/issues/update_service_spec.rb
@@ -491,6 +491,27 @@ describe Issues::UpdateService, services: true do
       include_examples 'updating mentions', Issues::UpdateService
     end
 
+    context 'duplicate issue' do
+      let(:canonical_issue) { create(:issue, project: project) }
+
+      context 'invalid canonical_issue_id' do
+        it 'does not call the duplicate service' do
+          expect(Issues::DuplicateService).not_to receive(:new)
+
+          update_issue(canonical_issue_id: 123456789)
+        end
+      end
+
+      context 'valid canonical_issue_id' do
+        it 'calls the duplicate service with both issues' do
+          expect_any_instance_of(Issues::DuplicateService)
+            .to receive(:execute).with(issue, canonical_issue)
+
+          update_issue(canonical_issue_id: canonical_issue.id)
+        end
+      end
+    end
+
     include_examples 'issuable update service' do
       let(:open_issuable) { issue }
       let(:closed_issuable) { create(:closed_issue, project: project) }
diff --git a/spec/services/quick_actions/interpret_service_spec.rb b/spec/services/quick_actions/interpret_service_spec.rb
index a2db3f68ff74dad6b041cf3286a62c8246cf908a..2a2a5c38e4beca53b7fa8e5852134f9bc22e092d 100644
--- a/spec/services/quick_actions/interpret_service_spec.rb
+++ b/spec/services/quick_actions/interpret_service_spec.rb
@@ -261,6 +261,15 @@ describe QuickActions::InterpretService, services: true do
       end
     end
 
+    shared_examples 'duplicate command' do
+      it 'fetches issue and populates canonical_issue_id if content contains /duplicate issue_reference' do
+        issue_duplicate # populate the issue
+        _, updates = service.execute(content, issuable)
+
+        expect(updates).to eq(canonical_issue_id: issue_duplicate.id)
+      end
+    end
+
     it_behaves_like 'reopen command' do
       let(:content) { '/reopen' }
       let(:issuable) { issue }
@@ -644,6 +653,41 @@ describe QuickActions::InterpretService, services: true do
       let(:issuable) { issue }
     end
 
+    context '/duplicate command' do
+      it_behaves_like 'duplicate command' do
+        let(:issue_duplicate) { create(:issue, project: project) }
+        let(:content) { "/duplicate #{issue_duplicate.to_reference}" }
+        let(:issuable) { issue }
+      end
+
+      it_behaves_like 'empty command' do
+        let(:content) { '/duplicate' }
+        let(:issuable) { issue }
+      end
+
+      context 'cross project references' do
+        it_behaves_like 'duplicate command' do
+          let(:other_project) { create(:empty_project, :public) }
+          let(:issue_duplicate) { create(:issue, project: other_project) }
+          let(:content) { "/duplicate #{issue_duplicate.to_reference(project)}" }
+          let(:issuable) { issue }
+        end
+
+        it_behaves_like 'empty command' do
+          let(:content) { "/duplicate imaginary#1234" }
+          let(:issuable) { issue }
+        end
+
+        it_behaves_like 'empty command' do
+          let(:other_project) { create(:empty_project, :private) }
+          let(:issue_duplicate) { create(:issue, project: other_project) }
+
+          let(:content) { "/duplicate #{issue_duplicate.to_reference(project)}" }
+          let(:issuable) { issue }
+        end
+      end
+    end
+
     context 'when current_user cannot :admin_issue' do
       let(:visitor) { create(:user) }
       let(:issue) { create(:issue, project: project, author: visitor) }
@@ -693,6 +737,11 @@ describe QuickActions::InterpretService, services: true do
         let(:content) { '/remove_due_date' }
         let(:issuable) { issue }
       end
+
+      it_behaves_like 'empty command' do
+        let(:content) { '/duplicate #{issue.to_reference}' }
+        let(:issuable) { issue }
+      end
     end
 
     context '/award command' do
diff --git a/spec/services/system_note_service_spec.rb b/spec/services/system_note_service_spec.rb
index 60477b8e9ba59a2cc5d6d145d75c58a2362b5461..681b419aedfbb561058d8cf6e5ebed5e18dd4e20 100644
--- a/spec/services/system_note_service_spec.rb
+++ b/spec/services/system_note_service_spec.rb
@@ -1101,4 +1101,54 @@ describe SystemNoteService, services: true do
       expect(subject.note).to include(diffs_project_merge_request_url(project, merge_request, diff_id: diff_id, anchor: line_code))
     end
   end
+
+  describe '.mark_duplicate_issue' do
+    subject { described_class.mark_duplicate_issue(noteable, project, author, canonical_issue) }
+
+    context 'within the same project' do
+      let(:canonical_issue) { create(:issue, project: project) }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'duplicate' }
+      end
+
+      it { expect(subject.note).to eq "marked this issue as a duplicate of #{canonical_issue.to_reference}" }
+    end
+
+    context 'across different projects' do
+      let(:other_project)  { create(:empty_project) }
+      let(:canonical_issue) { create(:issue, project: other_project) }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'duplicate' }
+      end
+
+      it { expect(subject.note).to eq "marked this issue as a duplicate of #{canonical_issue.to_reference(project)}" }
+    end
+  end
+
+  describe '.mark_canonical_issue_of_duplicate' do
+    subject { described_class.mark_canonical_issue_of_duplicate(noteable, project, author, duplicate_issue) }
+
+    context 'within the same project' do
+      let(:duplicate_issue) { create(:issue, project: project) }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'duplicate' }
+      end
+
+      it { expect(subject.note).to eq "marked #{duplicate_issue.to_reference} as a duplicate of this issue" }
+    end
+
+    context 'across different projects' do
+      let(:other_project)  { create(:empty_project) }
+      let(:duplicate_issue) { create(:issue, project: other_project) }
+
+      it_behaves_like 'a system note' do
+        let(:action) { 'duplicate' }
+      end
+
+      it { expect(subject.note).to eq "marked #{duplicate_issue.to_reference(project)} as a duplicate of this issue" }
+    end
+  end
 end