It is. Different clients handle it differently. If you note, the repository where the request is made is for lemmy-ui, not lemmy backend.
E.g. Photon shows a single item with a dropdown to expand matching posts if they were posted at the same, which is common with cross-posts on All, Local, Subscribed feeds.
And then it lists all cross-posts on individual pages in a neat list with stats like votes, number of comments, etc.
Your proposal seems to target the same issues as with multi-community support https://github.com/LemmyNet/lemmy/issues/818, which just got 6000€ funding from NLnet. Which seems to be a cleaner way of achieving the same goal.
Some suggested points are also against ActivityPub standard.