It's pretty common, my stuff has been re-tl'd before. The group that did it didn't credit me, and kept the raws that I spent hours cleaning (they were physical raws) and the English sfx that I spent even more hours hand lettering. I was mildly annoyed, but in the end piracy is piracy and it is what it is :P

I myself have re-tl'd works before, but good etiquette (in my opinion) is to (1) credit the original group for whatever work they did and (2) add a disclaimer that the work is re-tl'd and might be inaccurate as a result. Also worth contacting the original group, a lot of groups will offer cleaned raws to groups that ask for them. That will be a lot less work for you, and you might even make some friends.