Just DLookup the SSN in the BeforeUpdate event, and then MsgBox asking if they want to add or not.

I have been trying for the past 5 hours to get this to work. Pleae help me write this one line code. I wrote:
Private Sub SS_BeforeUpdate(Cancel As Integer)
DLookup("CustomerF", "SS", "SS = " & Forms![CustomerF]!SS)
End Sub

Where the Form is called CustomerF and the field is SS. All I want is to this to notify me by MsgBox when the SS has been used with a current customer.  I do not want to stop someone from using the smae SS, just notify the user that he or she has entered  a duplicate SS number.  (Rarely we give the same customer 2 different accounts with the same demographics including the SS number.

Your parameters for DLookup are wrong.

1. Watch the DLookup video.
2. You have to put the value returned by DLookup into a field or a variable.
3. The first parameter is the FIELD you're looking up, not a form name. Never a form name. You look up from a table or query.
4. If SSN is a text field (and it should be) you'll need Double Double Quotes
5. I would go watch Access Expert 10 where I cover DLookup for the first time. You have it.

Put all of that aside for a moment. The EASIEST solution for what I think you're trying to do, is to just INDEX the SSN field in your table and set it to NO DUPLICATES. That will prevent a double entry. We can worry about the whole DLookup thing later.

Indexing

Problem is Rick, he wants to enable the creation of duplicate SSN's, but warn if there is a duplicate.

Practice the DLookup on something simple and follow along with the video. You'll get there

Ah. My mistake. Maybe I'll make a video out of this.

Dan, you are correct.  In this day and age of security all medical practices must conform to HIPAA regulations.  That not only means printed and digital security but other changes.  Our office administers to several different health care professionals with one front desk. Rarely patients are seen by more than one specialist. So therefore we wish only to have a warning when the SS# comes up duplicate. Then the office user can determine if that patient with the duplicate SS is being seen by another health care professinal or indeed there has been a duplication of that patient's records with the same physician. Thank you. I think that this would make an excellent video as using a DLookup to search in the same database can be very useful for a number of purposes.