We don't have a hard and fast rule about never reposting the same thing, especially if it's on topic for more than one forum. Different members may see it in each place and of course, respond from different points of view.

Some other ways might be to post more than one version, with some parts the same and other parts customized for a particular forum; or to put the full version in Forum A, then refer to it briefly it in forums B and C with a link to your Forum A post.

If someone were to post the same thing ten times, it would probably be redundant and/or off topic in at least some of the places where it was posted. We call that "flooding".