ci: fix duplicate pipelines

.gitlab-ci.yml

@@ -64,6 +64,9 @@ workflow:
           $CI_COMMIT_BRANCH =~ /(stable|develop)/ ||
           $CI_COMMIT_TAG
         )
+    # Never run for branches that have an open merge request
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
     # Run for merge requests from any repo or branches
     - if: $CI_PIPELINE_SOURCE == "merge_request_event"