Double firing is normally caused by a couple of things, but since you say it occurred since it was last serviced. I would take it back to whoever serviced it as they changed both the hop-up and opened the mechbox. So in this case it could be either of these and not something like a mag spring on steriods.
IF the hop-up is working properly it may lallow more than one BB through. This is easily checked by engaging more hop-up and seeing if the problem goes away.
Otherwise it is a mechbox problem
If the mechbox sounds like it is rewinding after a shot the antireversal latch may not be engaging properly and this will lead to allsorts of nice side effects like broken pistons. I've had M14 mechboxes do double firing before, as it seems they quite prone to timing issues unlike most other mechboxes. Also as the M14 mechbox has all the latches external to the box if they're not nice and freemoving they could also induce the problem, as they let the mechbox overrun-KART mechboxes and clones (UTG maybe?????) really like this if messed around.
This is easily seen by the feed nozzle doing 2 cycles back and forward per shot, as observed from the mag well.