Fix error when duplicate users are merged in approvers list

If an user who belongs to an approval group and also is an individual in
the approval rule, `MergeService` will fail in the `after_merge`, which
will cause `MergeWorker` to retry again. Since the merge has been
successfully merged, `MergeWorker` will encounter an error since
`MergeService` is not idempotent.

This change fixes the issue by using the Array |= join method to add an
element to the Array, unless it is already present. This fixes the
immediate bug, but we will have to address the idempotency issues later.

Closes https://gitlab.com/gitlab-org/gitlab/issues/13488

changelogs/unreleased/sh-fix-dupe-approvals-failing.yml

+---
+title: Fix error when duplicate users are merged in approvers list
+merge_request: 17406
+author:
+type: fixed

ee/app/services/approval_rules/finalize_service.rb

@@ -26,7 +26,7 @@ module ApprovalRules
 
     def merge_group_members_into_users
       merge_request.approval_rules.each do |rule|
-        rule.users += rule.group_users
+        rule.users |= rule.group_users
       end
     end
 

ee/spec/services/approval_rules/finalize_service_spec.rb

@@ -94,6 +94,20 @@ describe ApprovalRules::FinalizeService do
           expect(rule.approved_approvers).to contain_exactly(user3, group2_user)
           expect(subject).not_to have_received(:copy_project_approval_rules)
         end
+
+        # Test for https://gitlab.com/gitlab-org/gitlab/issues/13488
+        it 'gracefully merges duplicate users' do
+          group2.add_developer(user2)
+
+          expect do
+            subject.execute
+          end.not_to change { ApprovalMergeRequestRule.count }
+
+          rule = merge_request.approval_rules.regular.first
+
+          expect(rule.name).to eq('bar')
+          expect(rule.users).to contain_exactly(user2, user3, group2_user)
+        end
       end
     end
   end