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 ?
Yes, it can happen in theory.
However, the chances of it happening are much lower than "really low probability", more like "virtually impossible low probability".
Of course, as others have pointed out on this thread, if you are not using a good source of entropy or an otherwise flawed random number generator, the chances of it occurring rise. The reason is because the limited entropy and/or RNG flaws effectively lower the number of possibilities that it will ever generate.
To visualize this, for example, if a normal key to a house has 10,000 combinations that the "lock making machine" randomly assigns to doorknobs, you can look at a flawed RNG as if part of the lock making machine is stuck, so in reality it will only ever generate 1,000 different combinations.
Randomness is valuable. If using virtual machines to store wallets/run nodes/etc., ensure that they have access to a good source of entropy. VMs often have poor entropy without specific steps being taken to increase the levels (platform specific).
Best regards,
Ben