Flickering Error message: Check duplicates

I'm not sure what I have done ... nothing as far as I can tell but obviously
something.

I have used the Contacts management template as the basis of a Contacts/
Event database. A couple of issues have arisen - but the most annoying one is
the appearance of a flickering Error message (where a warning about possible
duplicates appears) in the Contact Details form.

The Properties are - Text box: txtDuplicatesWarning
and the code that comes up in the Expression Builder is:
=IIf(DCount("*","[Contacts Extended]","[ID]" & Nz([ContactID],0) & " And
[Contact Name] = '" & Replace(Nz([Contact Name]),"'","''") & "'")0,"Possible
Duplicate","")

ContactsExtended is a query that came built in with the template which I
haven't changed - though I have done some work on the Contact Details form on
which the error is displayed - adding fields from the Contacts table)

Is there something obviously wrong? The warning would be useful but the
flickering Error definitely isn't! If you need any more information, please
let me know!

Many thanks