Just like bit.ly is doing it. If you shorten a long URL without being logged in the script should verify if the same URL was asked to be shorten before and then if there's the same URL in the DB it should give you the same short URL, e.g.
If anonymous user wants to shorten the URL:
http://maps.google.co.uk/maps?oe=utf-8&client=firefox-a&rlz=1R1GGLL_en-GB___GB423&um=1&ie=UTF-8&q=google+maps+big+ben
&fb=1&gl=uk&hq=google+maps+big+ben&hnear=google+maps+big+ben&cid=0,0,7629721680134612
123&ei=AukATpvoBpDA8QPaq4mzDQ&sa=X&oi=local_result&ct=image&resnum=1&ved=0CCAQnwIwAA
Then the scripts returns:
http://gourl.us/a
If another anonymous user wants to shorten the exact same URL why the system bothers to create another short URL? it should return exactly the same short URL:
http://gourl.us/a
This should be for no registered users.
If anonymous user wants to shorten the URL:
http://maps.google.co.uk/maps?oe=utf-8&client=firefox-a&rlz=1R1GGLL_en-GB___GB423&um=1&ie=UTF-8&q=google+maps+big+ben
&fb=1&gl=uk&hq=google+maps+big+ben&hnear=google+maps+big+ben&cid=0,0,7629721680134612
123&ei=AukATpvoBpDA8QPaq4mzDQ&sa=X&oi=local_result&ct=image&resnum=1&ved=0CCAQnwIwAA
Then the scripts returns:
http://gourl.us/a
If another anonymous user wants to shorten the exact same URL why the system bothers to create another short URL? it should return exactly the same short URL:
http://gourl.us/a
This should be for no registered users.