Issue on GIT branch Protection Rule

Enabled protection rule to a branch such that only certain users can merge others code, but not their own code.

Developer 1 creates a pull request and sends for an approval for merge.
Now Approver modifies the code on the same pull request, now the approver has the rights to approve/merge the pull request which should not be the case
How to avoid this issue?
The approver shouldn’t change the code when there is an pull request already opened by other developer.