Often, people ask the same question on multiple StackExchange websites. While this might be sometimes fine (a question did not get enough attention, the posts are cross-linked, some time has passed from the initial question submission - see CS Discussion on Cross-posting), there are a lot of obvious cases when a question is asked simultaneously at all StackExchange websites.
I suggest adding as a standard closing reason.
Closing -> CrossPosting -> Enter a question link or numeric id
Closing -> Duplicate -> Enter a question link or numeric id and allow for non-CompScience links.
Example of CS question, for which one has to type a custom reasoning for off-topic for a question that is certainly crossposted to CS and SO.
Or does one has to flag such post
in need of moderator attention?