"Started translating again"
Roll Eyes.
Isn't the spirit of Scanlation supposed to be to allow people to read manga that they otherwise would not be able to? In that vein, there is no reason two different groups cannot do separate translations of a manga. People can then just read whatever Scanlation they prefer.
The only 'rude' thing I can see happening is that if Group B releases something that Group A was working on.... Group A could feel that they 'wasted' some time and effort. However, if your goal is actually just to facilitate more people being able to read something then I can't see why anyone could be 'angry' about that. The groups can either try to collaborate for faster releases, or each just do their own thing and let people read whichever suits their fancy more.
Scanlation isn't legal anyway, so no one has the right to actually call 'dibs' and say they are the only people that can be translating the Manga. You'd need to be a legitimate publisher to have any right to tell people 'No! WE are the ones who will translate this!"
Excluding any mention of 'quality', the only other reason a group could be upset about someone else also Scanlating something is that people may read that other group's Scanlation instead of their own. And if that is their Primary reason for Scanlating something, then I say that group can be ignored.