@SaschaH
Then you probably have some records that were not properly deleted and I assume the server has been restarted or at least MySQL so the auto-increment value was set to the last available value which matches an old record that was not deleted properly. This has been explained in many other threads like:
You can only upgrade to MySQL 8.0+ and cleanup duplicates as you go along. Once you've cleaned all duplicates and upgrade to MySQL 8.0 this shouldn't happen again as the auto-increment value will be persisted instead of restarting at the last available value.
Here is a good read on the MySQL issue:
Cheers.