fix multiple notifications from being sent for multiple labels

This also refactor the email_helper support spec to watch for multiple
emails being sent.

app/services/notification_service.rb

@@ -397,7 +397,7 @@ class NotificationService
   end
   def relabeled_resource_email(target, labels, current_user, method)
-    recipients = labels.flat_map { |l| l.subscribers(target.project) }
+    recipients = labels.flat_map { |l| l.subscribers(target.project) }.uniq
     recipients = notifiable_users(
       recipients, :subscription,
       target: target,

spec/services/notification_service_spec.rb

@@ -743,6 +743,18 @@ describe NotificationService, :mailer do
         should_not_email(@u_participating)
       end
+      it "doesn't send multiple email when a user is subscribed to multiple given labels" do
+        subscriber_to_both = create(:user) do |user|
+          [label_1, label_2].each { |label| label.toggle_subscription(user, project) }
+        end
+
+        notification.relabeled_issue(issue, [label_1, label_2], @u_disabled)
+
+        should_email(subscriber_to_label_1)
+        should_email(subscriber_to_label_2)
+        should_email(subscriber_to_both)
+      end
+
       context 'confidential issues' do
         let(:author) { create(:user) }
         let(:assignee) { create(:user) }

spec/support/email_helpers.rb

 module EmailHelpers
-  def sent_to_user?(user, recipients = email_recipients)
-    recipients.include?(user.notification_email)
+  def sent_to_user(user, recipients: email_recipients)
+    recipients.count { |to| to == user.notification_email }
   end
   def reset_delivered_emails!
@@ -10,17 +10,17 @@ module EmailHelpers
   def should_only_email(*users, kind: :to)
     recipients = email_recipients(kind: kind)
-    users.each { |user| should_email(user, recipients) }
+    users.each { |user| should_email(user, recipients: recipients) }
     expect(recipients.count).to eq(users.count)
   end
-  def should_email(user, recipients = email_recipients)
-    expect(sent_to_user?(user, recipients)).to be_truthy
+  def should_email(user, times: 1, recipients: email_recipients)
+    expect(sent_to_user(user, recipients: recipients)).to eq(times)
   end
-  def should_not_email(user, recipients = email_recipients)
-    expect(sent_to_user?(user, recipients)).to be_falsey
+  def should_not_email(user, recipients: email_recipients)
+    should_email(user, times: 0)
   end
   def should_not_email_anyone