I'm playing with raw transactions. It's easy to send two signed valid transactions at the same time from the same output to different addresses (making a double-spend attack). My doubt is: if I make tests like this, will the bitcoin p2p network will flag my node / IP as "bad" or I can do it normally without any consequences? Of course I am not attacking anyone else, I want to make tests with my wallets / system to see the behavior.
No, spending an already spent output is not a DoS banning offense. Sending invalid transactions will however get you banned from your peers.
Your current peers can ban (for some time, not forever) your IP-address. Just connect to other nodes in case. The network itself can not do anything with you because there is no such thing in protocol. Your peers do not know - are you a double-spender or you just relaying somebody's else packets.
(Sorry, my English is also very poor)