One person posting the same thing to multiple communities is bad etiquette, whether the stated purpose of those communities is the same or not. Anyone who does this should be shamed, blocked and banned. Find the biggest and most relevant community and post it there, don't spam.

I focus on comments rather than posts, but if you care about finding new links then I agree that duplicate communities is a problem. But it's not the biggest problem on the platform, and obviously while instances are independent there's no way to force a solution. Communities have merged, but it's usually small communities on small instances dying as their users move to a large community on a large instances. This is a disaster for decentralization, everything needs to be as spread out as possible across instances.