Just to clarify, when the submit button is clicked, the duplicates search
would look first select all records in the current event code, then list
duplicates for those records with the same first and last names.

You can check if a DCount(...)  returns 0 or something else (in that case,
there is already a record with the given criteria.

   Dim strCriteria AS string
       strCriteria="FirstName=FORMS!formNameHere.FirstName "
       strCriteria = strCriteria + " AND LastName =
FORMS!FormNameHere.LastName "
       strCriteria = strCriteria + "AND Event=FORMS!FormNameHere.Event"

       if 0 <> DCount("*", "tableNameHere",  strCriteria) Then
               ' Asking first if the user want to open a form to edit
existing records
               ' if so ...
               DoCmd.OpenForm ... , where:= strCriteria   ' note the :=
syntax
                                                                       '
when using a named argument

                if 0 <> DCount("*", "tableNameHere",  strCriteria) Then
                   'there is still dup, what to do?  eliminate this
proposed row not entered yet?

                   Cancel = true
               end if

       else

           'do nothing

       end if

Note that DCount counts the rows already committed in the database, not THIS
(assumed NEW)  ONE. There are still a couple of things to do, such as asking
if the end user want to edit previous records. If the end user does not what
that, or if, after having edited it, there is still a dup, what we do?  I
assume this code is somewhere where you can 'CANCEL'  the process to insert
this new record in the database, but clearly,  something else can be the
proper way to do it, in your case.

Hoping it may help,
Vanderghast, Access MVP