Gridview update mode Validation

Question

Plz help Me

I have a gridview and i has edit button, when updating the edited field i checked for the duplication of the record and if the data gets duplicated it should show error msg

when i first enter for duplication of data it would not show me the error but second time when i enter it will show the error msg it means two record can contain duplication of data but not more

 

its the name i want to check for duplication of data