I am not ready to accuse, but I wanted to bring something up that I feel shouldn't be allowed. I have played a doubles game with a duo. I checked on the games they are playing and I noticed that they play a lot of team games together (doubles, triples), and they also seem to be together in all singles games that that they play. if they are colluding, it would give them an unfair advantage to have a built in ally everytime you play a singles game.
From looking at their games, they almost never attack each other, maybe once per game. I don't know if it's a multi or secret diplomacy.
I've been in games where I played singles games against people that I later took on as teammates. After we became teammates, I never played singles against them anymore because I felt it would prejudice us to treat each other better than the other players. I also recently played in a game where the other two remaining players solely attacked me even though I wasn't even the strongest player and sometimes attacking each other would have been the only thing to do. They treated each other like allies and left their borders unprotected against each other. I've tried reporting players that I thought were cheating but I feel CC's system for investigating and punishing cheaters is too weak. So all i can do is FAMO and always keep a look out for other cheaters.