[Dynamo]

I would say it's either your hop-up or your anti-reverse latch isnt correctly installed. There isn't really anything to time as far as gears.
Maybe your spring is too soft for the rate of fire you got, and it doesn't have enough power to stop the gears from rotating.
I'm really leaning towards a hop-up issue since the gearbox should only draw the nozzle back during the pull back of the piston, and release the nozzle befor the piston is released.

So if there is a problem with the hop-up, it may let two BBs in everytime the gearbox cycles. The BB should not be able to move from the feed tube forward into the hop-up without the nozzle pushing it forward. Check your hop-up sleeve. Remove the barrel and hold it with the hop-up pointing up, then place a BB on the hop-up openning. The BB should not just drop in, but it should be held in place by the hop-up openning. If it drops into the hop-up with out you pushing it (lightly) with a finger, then your hop-up openning is too wide.

If there is nothing stopping the BB from moving forward, then the BB below it will push it with the tension from the mag spring thus the double feed.