How to invoke a Servlet or java method onSubmit ?

I am creating a Form in a Servlet.

e.g

<FORM ACTION="/servlet/addUser" onSubmit=? >

Lastname <INPUT TYPE ="text" NAME =lname >

Firsnname <INPUT TYPE ="text" NAME =fname >

<INPUT TYPE=submit name =submit value ="submit">

Now on hitting the submit button I need to check if a duplicate lastname exists in the database by invoking a servlet method and only if a duplicate does not exist I want to submit the form .

Since as I understand onSubmit can only invoke a Javascript method , how can I really invoke a servlet method to check for duplicate before addUser is invoked.

Thanks

S

[674 byte] By [sudipk] at [2007-9-26 7:52:44]