In my experience, it's RARELY necessary to replace the laser. Usually you can take a "non-functioning" laser and put it in a DuoR and it works perfectly fine. The problem, other than the capacitor replacement that's well-documented, is usually the pots on the Duo. You'll definitely need to adjust them to get the best play experience.
I meant to post pics of it, but I have a way to adjust the pots during disc play to make the process MUCH easier than removing the top, turning a bit, replacing the top, testing. I simply drilled holes in a crappy Duo cover I had and can adjust the pots during play. It works fantastic. Can get pretty much any Duo playing any game including burns now. Someday I'll post the (obvious) pic of it.