Any post and community could be accessed through a theoretically limitless amount of instances, which also means a theoretically limitless amount of URLs.
Will this hinder Lemmy from ever coming into the mainstream? If I type any topic in Google, I will get a reddit thread that deals with that. Can something like that ever happen for Lemmy?
The canonical makes sense for the search engine (eg Google). I would put the canonical on the source instance.
Leaves open the question what would happen if the source would disappear…