What is the proper Netiquette?

I personally think double posting is worse than cross posting. If you can't justify cross posting, you probably shouldn't double post. If you think it should be in both groups, then just cross post correctly.

You could also post to one group, and when the discussion ends, and you feel you still want to see more discussion, post to the second group. After the discussion in the first group, you will probably end up posting something slightly different in the second group anyhow.

There's also the option of setting the Followup-To: header to have all the follow-up discussion go to a smaller set of groups. That seldom works well in my opinion because if you aren't willing to have the discussion in one of the groups, you probably shouldn't have bothered them with the first post. It works best for announcement posts sent to many groups where you never intended there to be a follow-up discussion.

I see this whole newsgroup thing as a good source of information. If you have a question for, or are offering information to the folks in different NGs, then by all means crosspost. The only way I'm annoyed is if someone is trying to bait me into buying something (spammers), and that is pretty easily dealt with. Simply go to the next topic. Two knobs on the radio, you know?

My $0.02.

I think crossposting is fine if you restrict yourself to two or three groups. Only if you are announcing something of interest to a large number of groups should you post to more, and then you should be asking if you're posting spam.

Don't multi-post. It's a pain in the behind for everyone.

Here's a good starting point

--Curious! I prefer double posting because it gives me a chance to tweak the text to the specific focus of the newsgroup. It means more work for the poster, but then you're more likely to get the response that you need.