If you see the message "you have already joined the table" when launching a game. It means that you already have joined this table on another device or on another browser tab and are you are trying to join a specific table for the second time. Since, you can only be at a given table once (otherwise you'd be playing against yourself), you need to find and close the other game (or close this one and continue playing on the other one).
To avoid this issue, it's always better to launch games from the game launchpad, instead of using bookmarks of old games.