You will need to modify the code of that page and add a function for validating the email and checking against a duplication.  Sample code would look something like this:

Function check_equalto(original, repeated)

 check_equalto = false

 If (original = repeated) Then
  check_equalto = true
 End If

End Function

If (not check_equalto(FTGReenter, FTGEmail)) Then
 validationFailed = true
 FTGReenter_errmsg = "Email addresses do not match"
 errorList = errorList & FTGReenter_errmsg & "<br/>"
End If

That code is being taken from a seperate application.  Digging into the source code of the registration.ascx or manageuser.ascx pages in the solution file would be the way to see how DNN is doing its validation.  From there you could just copy the behavior from the password validation to an email validation.