If/When Lemmy and other federated services grow to the point that’s an issue in major search engines, said search engines should be smart enough to group and/or suppress mirrored results.
You can see that sort of thing in Google now for major sites like Reddit and StackOverflow, though it’s more along the lines of “the same question in a different post”.
You can also, in the interim, just pick an instance and add, site:lemm.world or whatever instead of just “lemmy”.