When trying to set up a new mirror, an admin needs to approve the mirroring request on the primary server. If The user double- or triple- clicks the "Approve" button, the REST request to accept the mirroring request is sent multiple times in quick succession. The backend does not handle this gracefully and this can result in issues with mirror installation.
The frontend should disable the Approve button once it has been clicked once.
This issue has affected a customer running 5.3.x, and I've been able to reproduce it on our latest version as well.