Make it impossible to approve a pull request when you've commited to that pullrequest.
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We use branch protection. (Same user cannot approve it's own pull request) At this moment I can grab a pull request from a teammember, make additions, push and approve that pull request.
To prevent this, once a commit by user x has been made to a pull request, he should't be able to approve the same pull request.
Andrew Stephens commented
The problem to overcome with this is you may quickly run out of people that can approve the PR.