I mean.... there really is no reason multiple groups couldn't each do their own scanlation of a Manga.
Excluding 'maliciously intended sniping or stealing', If more then one person wants to give their take on a particular Series ... why not! At most you could be inconsiderate of effort put in by the other group if you release something they were currently working on (by making them feel like they 'wasted' time and effort). But if those two groups just both want to do their own thing (especially if one of them doesn't post their scanlations on MangaDex at all) then they really don't have any affect on each other. Readers are free to read whatever they prefer.
Now if someone gets mad that "less people will read our version!"....... They don't have any right to complain.
If its due to ego that they think they should be the only people working on it, they should grow up.
If its due to them being concerned about a loss of 'site traffic' or 'donations'...... I don't have many nice words to give them. Scanlation isn't legal, its illegal for them to be making money off it, or they need to recheck what 'donation' actually means.
Scanlation is supposed to exist to allow people to read Manga/Light-Novels that they otherwise cannot read as no official translation existed. If more then one group wants to get a Work into the hands of the masses.... they either work together to do it faster, or they don't and each just release there own.
Unless you repackaged someone else's Scanlation as your own (or some other Malicious crap just to screw with a group with no intention to actually keep releasing the serries), then there is nothing anyone should really be having any drama over....
Let Scanlators that want to Scanlate, Scanlate!
(Side note, especially if there is a lot to redraw..... i'd say it will normally take a couple days to prepare a Scanlation for release. The Raws for CH26 have been available for almost a month, and 'The other group' has some notoriety on sleeping on series for months, to say the least. So its likely coincidence that 'The Other Group' released a CH26 just before [NRBL], or (tinfoil hat time) someone tipped them off that [NRBL] was interested in picking this up so they got off their asses and finished something they could have completed much faster..... (again, that last bit is just conspiracy theory))