I tried shortening the same URL two times and it's giving me two different short URLs, have you considered checking the database to know if the long URL is already present, then you'll return the already shortened old URL

Secondly, this is more of a suggestion than an issue, I believe it'll be better if you don't display all the shortened links, maybe just display the one done by the user, because very soon your database might be having up to a thousand links and you won't want the user to be searching through those links...

I'd love to contribute to this but I've already done a URL shortener before, so I'll just check to see if there's any other project I can contribute to...