I think so i didn't understang good that you said :) but i've do it like below

* textbox properties lcuniq .t.,lctable (none),lcindex (none)
* textbox lostfocus event
If This.lcuniq
	If Thisform.EditMode=.T. Or Thisform.AddMode=.T.
		If Indexseek(Trim(This.Value),.F.,This.lctable,This.lcindex)
			Messagebox('This Code Allready Exist On Table',16,'Opps',2)
		Endif
	Endif
Endif