Check a record before its inserted

I am writing a contacts web app. The data will contain names and addresses of people. Inserting new names is done via a SQLDatasource and a Formview. I would like to write some code to assist in preventing duplication of names. Basically a check would be done before the new name is inserted to determine if it already exists. If it does, the user would be presented with the potentially duplicate record and asked if he/shee wanted to proceed with the insertion or abort it. My problem is this type of user interaction would normally be done via javascript but I don't know how to read the database with javascript or if its even possible.
Doing it by transporting the user to a new page with the question would mean losing the data he intended to insert.  
What would be the best way to approach this problem?
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