4chan

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 usually ignore that kind of thing. If it's excessive (5+ communities) and the user does it habitually, I consider blocking.

Some frontends, like PieFed's, are smart enough to collapse crossposts into one entry, showing only the post with the most engagement.

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.

TL;DR: get used to it.

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.

Seeing double posts is IMO not frequent enough to require mechanisms to fix it (and I can't even imagine a built-in mechanism against it).

c/greentext should be blocked because it's full of annoying fake stories, though.

It's something that the lemmy devs really ought to fix.

Scroll past the second one you see. Or if you never want to see a possible double post, then block one of the communities.