Even without double turns all you're waiting for is someone to START their next turn, so if you're quick you can still press/create an advantage equivalent to an actual double turn. This strikes me as not consistent with the spirit of the double turn prohibition.
Okay, I agree with you on this. But what if the other players can attack and only end their turns 5 minutes before the end of round (locking you out for the round)?
I also think that a (deadbeat) player missing a turn shouldn't count as the last player in a round. At the moment if you are the last active player who takes their turn in a round, you can effectively get a double turn - because the deadbeats turn will always come after you.
To me, a double turn was when your turn triggers a new round and you go again immediately. That's not the same as a you->round expires->you move, which was not blocked. But I guess that players who wait untill 5 mins before end of round can effectively benefit from a double turn.
So, how's this for a solution? The last player to begin a turn in a round (whether or not everyone played that round) cannot take a second turn untill either (a) another player ends a turn or (b) 12 hours have passed.