Avoid duplicate CI pipelines running at the same time

Docs:
https://docs.gitlab.com/ee/ci/yaml/README.html#switch-between-branch-pipelines-and-merge-request-pipelines
https://docs.gitlab.com/ee/ci/yaml/README.html#avoid-duplicate-pipelines

.gitlab-ci.yml

@@ -5,6 +5,15 @@ stages:
   - optional_tests
   - check_reproducibility
 
+workflow:
+  # when to create a CI pipeline
+  rules:
+    - if: '$CI_PIPELINE_SOURCE == "merge_request_event"'
+    - if: '$CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS'
+      when: never # avoids duplicate jobs for branch and MR
+    - if: '$CI_COMMIT_BRANCH'
+    - if: '$CI_COMMIT_TAG'
+
 .base-test:
   before_script:
     - set -e