One day you set up some service, for instance like this spam slayer setup. Later arises the situation when the distribution package use user account named X (for instance debian-spamd) while you set up things to use another one (for instance Debian-exim).
The easiest fix is to give them a common uid:
usermod –non-unique –uid 101 debian-spamd
(101 being user id of Debian-exim account).
Obviously it could be best to review the setup to really use two separates accounts. But it’s up to you to decide whether it’s changing it.
You may also add –gid too.