It’ll happen if Lemmy gets big enough. I only worry about search engines getting tangled in the natural duplication of Lemmy posts.
Like, if a web crawler sees a Beehaw post, and then seees Lemmy.ml’s mirrored page of that same post, could it just show up as two different results? Could it work against the SEO in that it gets marked as “duplicate” or “spam” content in some way?