Correctly remove already-mentioned commits for cross references

Fixes #1778

app/models/concerns/mentionable.rb

@@ -67,7 +67,13 @@ module Mentionable
   # Create a cross-reference Note for each GFM reference to another Mentionable found in +mentionable_text+.
   def create_cross_references!(p = project, a = author, without = [])
-    refs = references(p) - without
+    refs = references(p)
+
+    # We're using this method instead of Array diffing because that requires
+    # both of the object's `hash` values to be the same, which may not be the
+    # case for otherwise identical Commit objects.
+    refs.reject! { |ref| without.include?(ref) }
+
     refs.each do |ref|
       Note.create_cross_reference_note(ref, local_reference, a)
     end

spec/models/concerns/mentionable_spec.rb

 require 'spec_helper'
 describe Issue, "Mentionable" do
-  describe :mentioned_users do
+  describe '#mentioned_users' do
     let!(:user) { create(:user, username: 'stranger') }
     let!(:user2) { create(:user, username: 'john') }
-    let!(:issue) { create(:issue, description: '@stranger mentioned') }
+    let!(:issue) { create(:issue, description: "#{user.to_reference} mentioned") }
     subject { issue.mentioned_users }
     it { is_expected.to include(user) }
     it { is_expected.not_to include(user2) }
   end
+
+  describe '#create_cross_references!' do
+    let(:project) { create(:project) }
+    let(:author)  { double('author') }
+    let(:commit)  { project.commit }
+    let(:commit2) { project.commit }
+
+    let!(:issue) do
+      create(:issue, project: project, description: commit.to_reference)
+    end
+
+    it 'correctly removes already-mentioned Commits' do
+      expect(Note).not_to receive(:create_cross_reference_note)
+
+      issue.create_cross_references!(project, author, [commit2])
+    end
+  end
 end