~ my first unicycle, a 20" Torker LX, lost the right pedal after two weeks of riding…found out later that the lbs I bought it from had installed the wheel backwards in the frame.
~ my most recent unicycle, a 36" Nimbus from UDC, arrived with cranks on correctly but the little stickers that tell me which pedal is left and which is right were wrong. After wondering why I couldn’t screw the pedals in I tried installing them on the “wrong” cranks and they screwed on easily. I looked more closely and saw the L and R stamps on the pedal screw ends and knew I had them on right, (and the packagers had the helpful little stickers on wrong)
Cheap version: Weld in a new (or used), crappy pedal.
Expensive version: New crank, new pedal.
Yes, check to see your other crank is on the side it’s supposed to be on. If not, that explains the problem. If it is on the right side, you can also check the threads on your stripped pedal to see if they’re the right kind. It’s possible you ended up with two of the same pedal and/or crank.
A second possibility is a pedal that wasn’t tight enough, though they tend to tighten themselves when ridden forward. If it’s loose for a long time and you don’t notice, a lot of damage can happen relatively fast.
For the guy with the chronic Coker stripping problem, if you’re replacing cranks but not pedals, it could be a bad machining job on the pedal in question. If the threads are a little to thin or small, the pedal may ride just a tad loose in the crank and kill the threads after a while.
Otherwise, maybe you’re just a leadfoot on one side?