I've encountered a couple of times, where some players were suspiciously playing in both teams, but can't prove which the other one might be. It happened while our "teammate" had a large amount of troops, where he didn't had any enemy borders, yet also kept them on his territory after defeating the enemy. But he only attacked after a couple of turns later, but all that time, he had a chance of succeeding, but didn't made an action.

Yet he also had a territory next to mine where he had higher troops (40) than our common enemy border (0). I couldn't attack, because I had other borders that were more important than the one I had next to his. But he didn't made a move, and yet at the next turn, he just kept them there aswell, and the enemy kept on 0. Following turn, someone moved a large amount of troops there, and still didn't made a move.

After a couple of turns he lost it, even though it was an important territory to hold onto, because it could give a larger amount of troops than the other territories.

If you have a reasonable suspecion a player is multi accounting, you can report that player (from one of the games he is multi accounting in). In your report, you should include all the evidence you have for the cheating.

Please note that some players just have no idea how to play, so making super bad moves for multiple turns might just be a lack if practice and/or skill.