Dedupe cross-posts

[vcarl]

We recommend against cross-posting, it would be great to have it enforced by a bot. If somebody posts exactly the same message in more than 1 channel within a narrow time frame, Reactibot should remove one of the messages.

[nickserv]

@nickmccurdy People pinging other channels for help is a related issue that we should also enforce

[nickserv]

It seems like this is less common with our CoC gate and improved moderation in general. I'm not against closing it until it becomes an issue again.

[Simplyalex99]

See #460 (comment)

[achyu-dev]

This can be deleted probably, as another duplicate issue's PR already solves it