But what happens to the transaction if the same txid already exists? Cause it can’t be 100% impossible of creating transaction with the unique txid, can it? This transaction will be be waiting in the mempool for the moment when existing txid will be deleted from the LevelDB?
It is possible that there could be a txid collision (and in fact they have occurred due to a specific oversight in the protocol that has been fixed), but normally they are extremely unlikely because like private keys, they are 256-bit random numbers. However, collisions are avoided by rejected duplicate txids. I believe that this is correct and someone will correct me, if not.