Cross-post when necessary, but never multi-post. Cross-posting is when you
include two or more groups in the "Newsgroups:" section. In this scenario
the message and all responses are seen in all groups that the message was
cross-posted to. In other words any replies will automatically propagate to
the other newsgroup posts.

Multi-posting is when you post the same message to two or more groups
individually. In this scenario the message is seen in the groups it is
posted to, but the responses are only seen attached to the message (unless
you cross-post the reply) in which the response was made. So those that
frequent these groups then need to deal with or re-read the posts. It may
seem like a small thing to keep track of. But some of us actively traverse
50 or more groups. The poster also then must search out all of the posts to
see if there was a response.