On reddit you may have the same post twice but the comments will be different. On Lemmy, you have the same post on every federated lemmy with the same comments on all of them. With the way google handles websites right now, if they started including Lemmy instances in their web, it end up having hundreds of the exact same result each hosted on a different lemmy instance.
Edit for clarity: All lemmy sites share their data with each other unless they explicitly stop doing so (defederating). This is why I can respond to your comment even though I'm on kbin.social and you're on lemmy.world
Now I'm curious what would happen if I defederated from Kbin.social on kbin.social