I do not want to expose anyone, so I have censored the username. It is important to me that we are cool here and respect each other :)
It is only a guess, but I assume that the person unfortunately does not know what the fediverse is and how it works.
I'm not entirely sure which community it is, but I can make an educated guess, and wonder why he didn't do the same since it looks like there was already a bigger community for said topic.
But yea I don't see an issue with having multiples of one topic across an instance, especially when some instances have smaller groups or are more focused on a certain type of person, I mean federation means anyone can participate of course, but it's more likely the ones on the instance are going to if it's a smaller group.
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