Remove duplicates from issue related merge requests

Remove duplicates returned by Issues#related_merge_requests API that
relies on ReferencedMergeRequestsService which returns 2 arrays one of
related MRs and one of related MRs that close the issue(i.e. a subset
of first one). We only need related MRs in this case so just pick the
first array.

changelogs/unreleased/duplicate-related-mrs.yml

+---
+title: Remove duplicates from issue related merge requests
+merge_request: 27067
+author:
+type: fixed

lib/api/issues.rb

@@ -307,7 +307,7 @@ module API
 
         merge_requests = ::Issues::ReferencedMergeRequestsService.new(user_project, current_user)
           .execute(issue)
-          .flatten
+          .first
 
         present paginate(::Kaminari.paginate_array(merge_requests)),
           with: Entities::MergeRequest,

spec/requests/api/issues_spec.rb

@@ -2189,6 +2189,18 @@ describe API::Issues do
       expect_paginated_array_response(related_mr.id)
     end
 
+    context 'merge request closes an issue' do
+      let!(:closing_issue_mr_rel) do
+        create(:merge_requests_closing_issues, issue: issue, merge_request: related_mr)
+      end
+
+      it 'returns closing MR only once' do
+        get_related_merge_requests(project.id, issue.iid, user)
+
+        expect_paginated_array_response([related_mr.id])
+      end
+    end
+
     context 'no merge request mentioned a issue' do
       it 'returns empty array' do
         get_related_merge_requests(project.id, closed_issue.iid, user)