Thank you for the reply.
I guessed the probability is low, but didn't know if there is an "online address book" kind of protection on the online network anyway and if new public keys should be "reserved and activated" online before they can start receive coins.
An online address book wouldn't do any good. Remember spending goes both ways. If you generate a key which is in use (which we can safely say is so unlikely as to be infeasible) you could steal the funds of the existing keyholder (just as the existing keyholder could spend any funds you send there).
So you find out a key you generated already exists? What do you do delete the key? Ok so the system now relies everyone to be honest. Simply put if you EVER randomly generate an existing address, don't tell anyone. Sell every bitcoin you own and THEN tell everyone because it likely means some kind of catastrophic flaw in the cryptographic primitives. The infeasibility of not being able to duplicate a private key is a cornerstone on which the security model is based.
Having some kind of lookup and check would be as useful as having a "has the world been destroyed by a blackhole" app on your smartphone which will in realtime continually advise you that it hasn't happened. As long as the app shows a green checkmark you can safely know the world hasn't yet been destroyed by a rogue backhole.
