as you can see in this video: http://youtu.be/Q-AOr8dD2F4?t=29s
there is still a bug with retracting double-extenders.
while exploring a bug on bukkit, i found another one that was in vanilla too. even if no one uses this timings for a doubleextender, it is still a bug. it is consistent for each piston, als long as you don't change the redstone-current-strength.

configuration: the clock used for this is 8*1 tick long, 4 tick-repeater to a block, 2x1 tick repeater to bottom row of pistons, redstone-torch + 2 and 4 tick repeater to the top row of pistons, each piston has also a block to retract ontop.