that really does sound like the typical precocking tuning adjustment.
mosfets like the ab++/warfet run on timers, so essentially they wait for the cutoff lever to trip then add X milliseconds more runtime to the motor (which is what you set when you tune them).
if the delay is slightly too long, what can happen is the cycle settles in a different position each shot until that goldilocks position where it runs enough to fire twice. if your box has a window for viewing the piston you can test if this is happening by seeing how far back the piston stops each time, the behavior i'm describing it'll settle slightly further back with each shot until the double fire when it settles further forward and the process repeats.
having ab off with precocking can make things a bit more chaotic as motor overrun will happen naturally and can vary from just about everything you can think of (shimming tightness, motor, spring, piston tightness, hell even the grease viscousity or how warm the box is), indeed having too little overrun (typically active braking without precocking) is what give trigger lockups when the system stops too quickly. typically i'd advise ab and precocking together although if you're being conservative with how much precocking is going on it's probably ok to not have ab. you don't usually want ab without precocking unless it's a speediboi build that suffers double fires without it.
if you had the precocking really dialled in then i can absolutely see you going from perfection to just barely too long a delay just from a gear swap even if they're the same nominal ratio and shaving a tooth would absolutely do it.
sniper delay/dmr mode is just a rate of fire limiter, its only effect is to stop you firing a second shot if you spam faster than it's been told to allow, so shouldn't be affecting this.
that said, none of the above explains why it's happening with precocking turned off, but hopefully i've adequately explained why i'd want to be double/triple checking that it is definately off before testing for the behaviour. the other test is to change the amount of precocking, increasing it should make the doubles happen more frequently, reducing it should cure the issue.
