If Duplicate Values In A Field

I am looking for some help in writing an ASP script to check for duplicate values in a database. Using a web form to add and edit the records, the user will type/edit the data including phone number.

If the user types in a phone number or email address that is already in the database, I would like an alert box to indicate the name of the person with that phone number when the form is submitted. This is a more user friendly error instead of "Microsoft JET Database Engine error '80004005' ".