Sorry, that was incorrect. The code works only a regular index, not a primary or candidate key field. I never have a primary key field where the user actually enters the data. My PKs are always surrogate and integer that are generated.

If you want to do this, do not bind the control to a field. If you are going to do this regularly, create an ID textbox class with a cSource property:

local lcTable, lcField, lnSelect, lnRec, lnID, lnCount
lcTable = juststem(alltrim(This.cSource))  && mytable.fieldname
lcField = justext(alltrim(This.cSource))
lnSelect = select()
select (lcTable)
lnRec = recno()
lnID = This.Value
count to lnCount for ID = lnID
goto lnRec
if lnCount > 0
   ** Duplicate found
   select (lnSelect)
   return 0
endif
** otherwise the count was 0
replace &lcField with lnID
select (lnSelect)
return .t.