So, I had this question in my mind for a little time and now I think I really need an answer.
If you create a btc address you get the private key of that address, what if this address is already in use ?
I know there is a really low probability but this can happen, am I right ?
It
can happen if your wallet uses a faulty RNG (= random-number generator). That is, a biased / predictable RNG that results in a greatly reduced keyspace.
This happened back in 2013 and shows the importance of properly implementing security standards:
https://thehackernews.com/2013/08/hacking-bitcoin-android-vulnerability-digital-wallets.htmlShort of such software implementation failures the chance of a private key collusion is, as already mentioned by the other posters, infinitesimal.
Regardless of that you might want to check out the Large Bitcoin Collider that tries to put your worries to the test in practice:
https://bitcointalk.org/index.php?topic=1877935.0