Creates an actual idempotency spec

app/services/merge_requests/squash_service.rb

@@ -4,6 +4,10 @@ module MergeRequests
   class SquashService < MergeRequests::BaseService
     include Git::Logger
 
+    def idempotent?
+      true
+    end
+
     def execute
       # If performing a squash would result in no change, then
       # immediately return a success message without performing a squash

spec/services/merge_requests/squash_service_spec.rb

@@ -140,6 +140,13 @@ describe MergeRequests::SquashService do
     context 'when the merge request has already been merged' do
       let(:merge_request) { merge_request_with_one_commit }
 
+      it 'checks the side-effects for multiple calls' do
+        merge_request.mark_as_merged
+
+        expect(service).to be_idempotent
+        expect { IdempotentWorkerHelper::WORKER_EXEC_TIMES.times { service.execute } }.not_to raise_error
+      end
+
       it 'idempotently returns a success' do
         merge_request.mark_as_merged
         result = service.execute