Then block all other instances and keep the one that is most relevant, if you are that bothered with cross posts. Frankly, the main value of link aggregator sites are the comments, and having multiple instances can be great for making comparisons.

Yes, absolutely! Although, I don’t get this “there must be only one” mentality regarding communities. R*ddit had competing subreddits, since the organizational structure was ripe for mod abuse, and the community often made an alternative. Worrying about duplicate communities kinda misses the point of site federation.