I was playing with my bro. and we both were connected to a server and we spawned in same house where we disconnected before. He raised his AK (with his right mouse button) on me and gun started shooting and killed me. Then he put his gun down. After few seconds he raised his gun again using his right mouse button and gun started shooting again.
I was sure that he was not holding his left mouse button.
I have not tested it, but I think that happens only sometimes.