Duplicates, Editing , Dcount

Hi
I am trying to check for duplication with three fields while entering data.

I have set up a query on the recordsource query and dcount the records
this works in the before update code initially but when editing the record in any field
and moving off the record, I am incorrectly told my once correct entry is now a duplicate.
as though the record compares itself with itself.
ONly if I return its value to empty, move off the record and then reenter the field does it accept it as not duplicated

I'm confused and a little desperate.

thanks