They're not really "duplicate". Each instance has it's own magazines/communites/whatever, and those can have different rules, moderators, etc.
If you want to follow all of them, then follow all of them. If you only want one of them, then do one of them. Reddit had various subreddits all on the same topic, for instance /r/technology and /r/tech, and they had their own nuances, users, rules, etc. One might end up being more popular than the other for whatever reason.
Also keep in mind that they're subject to rules/moderation/control of the instance they're on. I'm using kbin.social, but if beehaw decides to do some weird thing or decide they no longer want a technology group, it's basically out of my control. Whereas a technology group here on kbin.social wouldn't be effected by beehaw's stance for their instance as a whole.
I imagine people interested in a topic will just follow all the related groups unless there's an issue with one of them.