Forms :: Custom Message For Duplicate Entries

I have tried the code below, but I am not getting what I need. I have a table [Student Info] with a field [Student ID] as the primary key and many other fields. I added the Student ID field to my form, [Student Add]. When I input an ID that already exists I get an error code as expected but then my form still loads. I currently have a second form for updating student info [Student Update] for which I have a parameter form that asks for the Student ID. I would like to use just one form to add and update students if possible. So, if you add a new Student ID you can enter all pertinent data. However, if you enter a duplicate Student ID the form should populate with that student's information.

If I must have two forms (1 to add and 1 to update) and a duplicate Student ID is entered into the Add form, the user should get a message stating that the record already exists. Then, (Option 1) force the Add form to close without saving or (Option 2) give an option for opening the Update form. Either option is acceptable at this point because I am stuck.

When attempting to use the code below, I get a data mismatch error on the "Answer = DLookUp......" line.

Here's one I use on Social Security numbers for a pop up when a duplicate....
SS# is stored in table "tblApplicant"

Private Sub SocialSecurity_BeforeUpdate(Cancel As Integer)
Dim Answer As Variant
Answer = DLookup("[SocialSecurity]", "tblApplicant", "[SocialSecurity] = '" & Me.SocialSecurity & "'")
If Not IsNull(Answer) Then
MsgBox "Duplicate Social Security Number Found" & vbCrLf & "Please enter again.", vbCritical + vbOKOnly + vbDefaultButton1, "Duplicate"

Cancel = True
Me.SocialSecurity.Undo