This will get you a list of those customers.  You could put this into a view and then query off of it.

SELECT

    COUNT(c.ID) AS DuplicateCount,

    c.LastName,

    c.Phone,

    a.Street

FROM

    Customers...

Steve,

I presume that you are using JavaScript to keep the user from clicking the submit button twice (double-clicking).  If the user has JavaScript disabled this will no longer be...