Hi Marijke

There are two  idnumbers in the user file - one that is automatically assigned and automatically increments - you can't change this

The other id number is used by the HR Import to uniquely identify users - this one can't be changed by the HR Import but can be changed by Upload users using the idnumber field and adding a value. 

It sounds like there been some user's deleted if this is the case and you don't want to keep any of the user data then you should purge the users data including the idnumber so it can be reused.

Regards