I'm new to Lemmy so I don't know if there's a good workaround for this, but OP posted this to /c/4chan as well as /c/greentext and both came to my feed one after the other.
Am I supposed to block OP for cross posting, block one of the two communities? Both ideas seem kinda stupid.
It's because OP made two separate posts and therefore uploaded the image twice so lemmy doesn't recognize it as cross-post (check the image URLs, they are different). If OP properly cross-posted it, the URL would be the same and you wouldn't see it twice in your feed.
If this is a one-time event, I would ignore & move on. If I ever see more than three posts in a row by a user, I block that user. It doesn't have to be permanent.
I don't know the details of how PieFed does it, but there is a button underneath whichever post is considered the 'main' one where you can view the details of other crossposts and visit them if you like.
If you think something is wrong if you see the same thing more than once, you're gonna have a bad time. Not only do things get crossposted between communities with different names, they also get crossposted between communities with the same name on different instances. Big political news often shows up a half-dozen times, or more.
I would just ignore it once or twice, I don't find it to be a big problem when browsing All. But if it becomes a repeat nuisance for you, since they seem to be two communities for basically the same thing, I would see if one is significantly more active than the other and block the less active one.