A few weeks ago, I started seeing multiple entries for Pixelmator in the “Open With…” area of my Finder’s contextual menu. And while I thought it was weird, I didn’t find it too annoying. So I just ignored it.
Just copy & drop that into your terminal app of choice and let it run. Once it’s all done, Control + Option + Click the Finder icon in your dock and select “Relaunch”. Once the finder comes back, “Open With…” should be all clean.
I’m mostly saving this here for my own benefit, but maybe it’ll help at least one other person out there.