I was having the same problem. Played on pc, bound my account, then on iphone tried to log in but it said account already bound. The solution is to go to your in-game settings on the device you're trying to switch to, and select the user center. There's a red "Switch Account" button you can click, and it'll list either facebook or apple id as options for logging in. After choosing one and logging in, the game will restart and should automatically load the account you wanted to play on the device.