It seems we found the reason, the server checks for a unique app name. This makes sense for official Apps, but is a bug for personal Apps. Thanks to karu for discovering this issue ;-)
It is not possible to add the same Name twice, but it is possible to add it with a random Name and later change it. To fix this, simply remove the check for duplicate names on custom Apps.
It is not possible to add the same Name twice, but it is possible to add it with a random Name and later change it. To fix this, simply remove the check for duplicate names on custom Apps.