I've come across a few popular, useful questions on Stack Overflow that have been closed as off-topic, which have duplicates on other sites. For examples, Stack Overflow question Is it possible to make a Bash file run as root in crontab? is closed, with a banner that suggests seeking help on Server Fault:
"Questions on professional server, networking, or related infrastructure administration are off-topic for Stack Overflow unless they directly involve programming or programming tools. You may be able to get help on Server Fault." – Duncan, Brian Clozel, Trinimon, Bhavik Ambani, AlSki
But there's already an open duplicate on another Stack Exchange site where the question is on-topic, How to run a cron job using the sudo command.
It would be nice if, rather than just closing the Stack Overflow question, somebody - even if the power was only available to moderators - could also mark it as a duplicate of the question on Ask Ubuntu. (The text of the duplicate banner might need to be slightly different from normal.)