Double firing at the same time
This is when both bumpers seem to launch at the same time. However what happens is that the second firing assembly has not engaged correctly, usually caused by a servo not returning correctly and blocking the lever from engage all the way. This can be checked by manually moving the lever to provide proper clearance and then cock the unit and retrying.
Check the servo for proper return position; it may be caused by loose wiring harness or a servo starting to malfunction
See Loose Wiring Harness (6MB video)
Replace Servo(13MB video)