Merge branch 'issue_13621' into 'master'

Don't repeat labels listed on Labels tab.

Fixes #13622 

See merge request !2924

app/models/milestone.rb

@@ -27,7 +27,7 @@ class Milestone < ActiveRecord::Base
 
   belongs_to :project
   has_many :issues
-  has_many :labels, through: :issues
+  has_many :labels, -> { distinct.reorder('labels.title') },  through: :issues
   has_many :merge_requests
   has_many :participants, through: :issues, source: :assignee
 

features/project/milestone.feature

@@ -13,6 +13,7 @@ Feature: Project Milestone
     Given I visit project "Shop" milestones page
     And I click link "v2.2"
     Then I should see the labels "bug", "enhancement" and "feature"
+    And I should see the "bug" label listed only once
 
   @javascript
   Scenario: Listing labels from labels tab

features/steps/project/project_milestone.rb

@@ -41,6 +41,12 @@ class Spinach::Features::ProjectMilestone < Spinach::FeatureSteps
     end
   end
 
+  step 'I should see the "bug" label listed only once' do
+    page.within('#tab-labels') do
+      expect(page).to have_content('bug', count: 1)
+    end
+  end
+
   step 'I click link "v2.2"' do
     click_link "v2.2"
   end