Simply set up a second query which just looks for the field
you want to check.

Post the value that is entered as a parameter and if a match
is fould set the error.

I think you could do this in the OnDataChange when the field
in question is changed, but there are probably better ways.
( I always use a pop-up and do the checks on exit from the
relevent edit box )

--
Lester Caine
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L.S.Caine Electronic Services