This comes across as the dreaded "JRA" failure. I'm not saying that you did anything, but stripping out two arms in a short period of time? Did you do something with the first arm/pedal and maybe it came cross to this one? I say give the shop a chance to help you determine what is causing this to happen. I'm guessing that it is something in the pedal if nothing else was touched after the shop installed it.

If the LBS does warranty this somehow, remember that and not only should you consider them "your source" but spread the word to other folks. It's hard for shops to deal with situations where the customer wants new product support from used or mail-ordered stuff. However, many will give it.

Report back.. I'm curious to find what the issue might be.