Update .gitlab-ci.yml file

.gitlab-ci.yml

@@ -10,6 +10,14 @@ include:
     ref: master
     file: 'jobs/common/master.yml'
 
+# See https://docs.gitlab.com/ee/ci/jobs/job_control.html#avoid-duplicate-pipelines
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
+
 before_script:
   - . /duneci/bin/duneci-init-job