It definitely isn't too common with smaller series but with some series, groups will release the same chapter within a day of each other, so it's not an entirely hypothetical problem.
In terms of implementation, the simplest method would be to just check for the various groups releasing for a title and let the user pick one which they want to be specifically notified about. It sounds intensive if you think about having to do it manually, but the data of which groups are scanlating which series is already available in the site's database (not only evidenced by the various releases under one title, but also on the Manga tab on group pages).
Implementing it will take some doing, sure, but once you've got it figured out it can just be applied automatically to all series. It'd be up to users themselves to actually set a preference for the feature to actually take effect.
Alternatively, you could implement it so that the site wouldn't notify users of chapters that are lower, or equal, to the current reading progress (a feature already implemented and automatically updated) but doing it this way would unfairly bias quicker groups, which is often not what you want.
While it's probably true that most people will often just read the first release available any way, implementing it so that later release will be completely invisible to users outside of actually going to the title's overview page would drastically cut down exposure of groups who release (possibly better quality) chapters later.
