find duplicate addresses in database, stop users entering them early?

Question

How do I find duplicate addresses in a database, or better stop people already when filling in the form ? I guess the earlier the better?

Is there any good way of abstracting street, postal code etc so that typos and simple attempts to get 2 registrations can be detected? like:

Quellenstrasse 66/11 
Quellenstr. 66a-11

I'm talking German addresses... Thanks!

Answer 1

You could use the Google GeoCode API

Wich in fact gives results for both of your examples, just tried it. That way you get structured results that you can save in your database. If the lookup fails, ask the user to write the address in another way.