First: Thanks Saturn

Well, I made sure the number didn't show twice. Then I checked addressbook.db with a hex-editor and found the addresses in question being there twice. Seems like some when the application crashed and left, well "leftovers".
So then I started:
- Backup - better sure than sorry
- Changed duplicate phone number to invalid one with a hex-editor
- Copied adderssbook.db back to N900
- Restarted N900 and looked at the contacts.
= N900 started rebuilding whatever - forever.
Hmmm
Looked at the filesystem and found under home/.osso-abook/db some db-rebuild folder. Seems N900 wanted to fix but either couldn't or I wasn't patient enough.
But ... the xxx.db file had now some initial size of 8192.
Good. So I reimported my V3.0 saved addresses and ....
EVERYTHING IS FINE NOW

Still don't know why the db had an error, but know now that saving (exporting contacts) is advisable.

Thanks everybody