Prevent duplicate pipelines

.gitlab-ci.yml

@@ -7,6 +7,13 @@ variables: &global_variables
   DB_HOST: postgres
   MIX_ENV: test
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == "merge_request_event"
+    - if: $CI_COMMIT_BRANCH && $CI_OPEN_MERGE_REQUESTS
+      when: never
+    - if: $CI_COMMIT_BRANCH
+
 cache: &global_cache_policy
   key:
     files: