Mattias,
  Not sure as to why you would wwant to do that but...

  If you don't declare NoDupes for a field in your table, you'll have to
manually validate each entry for that field.
  On the Before Update event for your field, you'll have to do a DLookup on
that value to make sure it doesn't exist in the table already.   If it
does... cancel the entry... if not, allow it.
  hth
  Al Camp