Multiple coins after currency fork Situation: bitcoin hardforks and I got payment with coins, that were in the blockchain before fork:

what can prevent me for sending the same transaction to the other forked network and get the second forked money, because I have a signed sending transaction and I own the target address private keys.
Nothing prevents you from doing such.  You now own coins on both chains, and can spend them independently. Assuming that adequate replay protection has been implemented, you shouldn't have to worry about your transaction on one network being broadcast without your consent on the other.