Question: A few seconds ago a first time user of AU asked a question (on AU), which was promptly and correctly [answered](https://askubuntu.com/questions/794013/what-is-in-terminal-command-line) by @terdon. That question had an [exact dupe](https://unix.stackexchange.com/questions/11376/what-does-double-dash-mean-also-known-as-bare-double-dash), equally well answered by @Guss, earlier on Unix & Linux, a SE communities with *some* overlap with AU and a few other SE communities.

Initially I was actually real surprised that said question was not already answered on AU; on second thought it actually *made sense* since it already had at least one excellent answer on Unix & Linux. 

It *made sense*, **but** I would not go as far as saying that before making a query on AU, one should first check whether there is an answer for it in a different SE community. Rather one should learn to post in the right community to begin with. Right, but sometimes it's not so easy to decide as in this case.

* So, how should one treat a query in a community A, which was already posted in unequivocal terms in community B, both on SE ?
* Should one call that a dupe or a "cross-dupe", without meaning that the two communities are one but somehow closely related ? I am not sure that sounds completely right.
* Should flair be awarded per usual, either through upvotes or automatic increment upon acceptance of such an answer to a cross-dupe ?
* Or should the query echoing another identical query in another community of StackExchange be treated differently in terms of flair accrual, categorisation as dupe, removal, etc. ?

Answer: 
### Short answer:

I believe cross-dupes not only are acceptable, but a small overlap is even useful:

* If the scopes of the sites overlap, so should their content.
* While a minor overlap does no harm *at all*, a *gap* between the sites would harm both.

For both sites being useful and complementary, provide an uninterrupted learning curve, the "travel" between them should be smooth.

A minor overlap is part of that. Imagine the situation that you constantly have to switch, because an (on topic) question was coincidentally answered on the other site.
Personally, I think that the best solution would be to develop a way of sharing questions across all sites where they're on topic. The rules against cross-posting were developed back in the days when there was very little overlap between the subject matter of different SE sites. However, things are very different today. The question you mention would be on topic here, on [Unix & Linux](https://unix.stackexchange.com/), on [Ask Different](https://unix.stackexchange.com/), [Super User](https://superuser.com/), [Elementary OS](https://elementaryos.stackexchange.com/) and maybe more. The current system would allow duplicate questions and duplicate answers across all of these sites. 

While I quite agree that this is less than optimal, it's the way things are at the moment. There is no way of flagging cross-site duplicates and no actions mods can take even if such a flag were raised. If you know that the same question has been asked and answered elsewhere, you can feel free to copy it verbatim and post it as an answer here. It would be polite to include a link to the site where you found it, of course, but copying SE answers is very much allowed. We had a discussion on [Unix & Linux meta](https://unix.meta.stackexchange.com/q/1396/22222) a while ago and that was the conclusion there as well. 

So, no, don't flag and yes, copy the existing answer. If you want to be super nice, you can make it [Community Wiki](https://askubuntu.com/help/privileges/community-wiki) so as not to get points for other people's work but, again, that's completely up to you.