idk if they an alt of someone as they both play pretty badly and if they are the same person or not you simply cant tell. you can only tell that they teamed. you gotta pay attention on how they do their turns. I would already say they arent the same person as people rather play games with high boot time so they have time to do their turns.
Yeah, no. IP's are dynamic, so yes, your IP changes and someone can have a IP you once had. But the chances that any two specific people ever had the same IP dynamically assigned to them is approximately 1 in 4 billion, that's how many uniquely identified devices there are via IPV4. If they have the same IP it should absolutely without question be assumed that they are the same person.
Teaming in FFA's is allowed, sure. But just because something is allowed doesn't mean you shouldn't blacklist people for it. Blacklist anyone you don't enjoy playing with. You don't need to violate the game's rules to be blacklisted.