Access 2000 – message boxes

The first field in my DB is a unique number, with no duplicates allowed (set in props.). But, the message that says “the record cant be saved as it will create duplicates”, appears when the user has filled in 50 or more fields. Therefore I would like to attach a message box to the number field in question, to say “If [field]= *the same as a previous record* then flag a message box, but as you can see i’m unsure of the syntax!
Can anyone help me please…