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.
I'm on the fence on this one more leaning toward the some overlap argument as a certain amount of overlap between here and U&L is unavoidable and likely positive, but Braiam does make an excellent point regarding due diligence. As it is, when I run across a question that I know has been answered (and answered well) on another SE site I just comment and point to the related answer rather than wasting valuable time re-answering. In fact if I'm doing MY due diligence I will discover the existing answer on another SE site resulting in the same action as if I had prior knowledge. Sometimes (often?) the question has been answered partially on a number of sites in which case I do my best to pull all the information together and cite the sources. I don't think different methods of determination or handling of cross-dupes are necessary.