I run my own gitea instance, and for repos that are in multiple places, I leave a note in the readme as to what the canonical URL is. If someone wants to open a PR or an issue on Github I don't wanna be hostile by having them create another account just to ask a question. Once the patch is merged, it gets pushed on all remotes.
If you have a large project with lots of contributors and activity, and you actively want to discourage contributions in the wrong place, you can disable issues or PRs for that one repo.
Also, people browse code using GitHub clients, serving an empty Readme is not a good idea to break that behaviour.
But when I go to a project on Github to report an issue and I can't, then I can't help but think "well fuck you too."
The main point isn't about the source code, it's about all the rest : bug report, discussions, planning..