Child Pipelines cause double Slack notifications on failure (or success)
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### Summary
When you use Child Pipelines, when the child changes state (eg fails) then the parent changes state (fails) and both are sent as Slack messages.
GitLab version: Current SAAS version (Dec 3, 2020).
### Steps to reproduce
- Configure Slack
- Create a child pipeline
- Cause it to fail
- See that two Slack messages arrive
### Possible fixes
One way to fix it would be to have a checkbox config on the Slack integrations screen to "Not send notifications for child pipelines".
issue