I don't see a problem with more than one group working on the same series in the same language either. They're probably doing this as a hobby, why shouldn't they work on what they want to? Unless someone's stealing your work and calling it their own most of the groups shouldn't care either, but I guess you can't underestimate the potential pettiness of people so I could see some silly crap happening there.
Also, if the first group to work on it was working with the magazine release, well. I can think of a ton of series I'd rather see the other releases of that series get handled eventually too even if another group does it. The tankoubon would probably have a bunch of extras, and for ecchi series it'd possibly be a less censored version. Chapters might have even been fixed since the magazine release, because something was rushed for deadline or whatever.