Filter out duplicates from the finder by related commit sha

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/21686

**Problem**

We discovered an edge case when the same merge request can be returned
multiple times. If the project has `always squash` option on, then we
create a squash commit even for merge requests with only one
commit. In this case, the commit from the merge request is a squash
commit at the same time.

**Solution**

Use UNION instead of UNION ALL to remove duplicated records.

app/models/merge_request.rb

@@ -270,8 +270,7 @@ class MergeRequest < ApplicationRecord
         by_commit_sha(sha),
         by_squash_commit_sha(sha),
         by_merge_commit_sha(sha)
-      ],
-      remove_duplicates: false
+      ]
     )
   end
   scope :by_cherry_pick_sha, -> (sha) do

changelogs/unreleased/21686_filter_duplicates.yml

+---
+title: Remove duplicates from related_commit_sha query
+merge_request: 51888
+author:
+type: fixed

spec/models/merge_request_spec.rb

@@ -451,6 +451,17 @@ RSpec.describe MergeRequest, factory_default: :keep do
 
       it { is_expected.to be_empty }
     end
+
+    context 'when commit is part of the merge request and a squash commit at the same time' do
+      let!(:merge_request) { create(:merge_request, :with_diffs) }
+      let(:sha) { merge_request.commits.first.id }
+
+      before do
+        merge_request.update!(squash_commit_sha: sha)
+      end
+
+      it { is_expected.to eq([merge_request]) }
+    end
   end
 
   describe '.by_cherry_pick_sha' do