Here is the issue: In order for Access to be able to know this, you will have two records, one of which exists already (in an incomplete state, but it has been stored.) The other hasn't been stored yet. The logic for detecting this has to have a "tell" somehow that lets it know that a pending record has not yet been completed but HAS been stored. It is not clear to me how you would do that unless someone tries to enter what WOULD be a duplicate ID if successfully entered. This would lead to a constrain violation that would trigger an error. If you don't handle the error correctly, you will crash the form, which is not too good a thing to have happen.
So my first question is, why don't you just prevent the first record (that is incomplete) from being stored in the first place. That way, the 2nd record NEVER becomes and issue. This is a case of (as the Old West cowboys would say) "heading it off at the pass." If you use the Form_BeforeUpdate to validate a record and it detects that the record is incomplete, you can cancel the update and thus never allow an incomplete record to be stored.