they do though

Edit: Ohhh, like it redirects there too not just links there

Yeah that would be a cool feature to offer the user when they submit

That is a bad idea. Cross posts aren’t a special type of post. Your instance simply checks the URLs of posts and finds other posts with the same URL. That is how cross posting works in Lemmy.

https://github.com/LemmyNet/lemmy/blob/6f45f801cc0582b14360fd52737cbad3a98b7250/crates/api/api_crud/src/post/read.rs#L90

[...]

  // Fetch the cross_posts
  let cross_posts = if let Some(url) = &post_view.post.url {
    SearchCombinedQuery {
      search_term: Some(url.inner().as_str().into()),
      post_url_only: Some(true),
      type_: Some(SearchType::Posts),
      ..Default::default()
    }

[...]