Always post in this format:
Thread title
(Blank line)
@Lemmy_community @Friendica_group/Hubzilla_forum/(streams)_group @Guppe_group (optionally more Guppe groups)
(Blank line)
Post body
this post was submitted on 01 Nov 2024
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Essentially, the title of the post/thread comes first.
Then comes a blank line.
Then come the mentions, all in one line. They must always start with the Lemmy community, and you can only mention one Lemmy community. If you want to crosspost to a Friendica group, a Hubzilla forum or a (streams) group, it comes next, and AFAIK, that can only be one, too. Guppe groups come afterwards, as many as you want.
Then comes another blank line.
And then comes your actual post.
Another blank line.
Lastly, hashtags so that Mastodon has them where it expects them.
Link to my post containing multiple group mentions and one Lemmy community:
https://chaos.social/@Draconic_NEO/113406243431431881
Link to post containing two Lemmy communities:
https://chaos.social/@Draconic_NEO/113406248999089892
FWIW: that post is not for 2 communities - it's for one community (/c/test at sh.itjust.works) and one user (/u/test at lemmy.ml) - I'm guessing that it's autocompleted to a thing that was different from your intentions.
(edit: the webfinger response from lemmy.ml for 'test' returns both a Person and a Group, which Lemmy can deal with, but Mastodon probably can't, so it just grabbed the first one it saw)
@andrew_s Ok that makes more sense but I do know I've tried it other times when there was no user and it posted to one community but not the other.
@lemmy_support
I'm guessing lemmy is not capable of crossposting the same thing to two different communities in general, the same way it's not possible via lemmy itself.
I'm not sure how it chooses which community to post to when there's multiple. I would assume it would pick the first one? If it's not, that, maybe it's random?
@db0 Might be, I can't be sure if it's just the first one, or random. I don't really want to try, my instance admin might not be too happy with me posting too many test posts in the name of science.
Though the weird thing is that it just straight up denies it even if the other groups aren't Lemmy or federate with Lemmy, which Guppe groups don't.
Ye I really don't understand how the same post in mastodon is possible to appear in two different groups, but not in lemmy. I am not familiar enough with the internals.
@db0 on Mastodon it just boosts the posts, I assume that's what Lemmy also does in addition to copying it and formatting the data as a post.
If I oversimplified or got that wrong I'd definitely appreciate if @dessalines or @nutomic could correct me on that. As far as I know there doesn't seem to be a reason why the same post can't appear simultaneously in multiple communties, since they're just boosts and duplicates on the federated instances. @lemmy_support