So there are two ways to do this:
- Close as a duplicate here, but instead of being restricted to the current site's questions when choosing a duplicate target, allow Stack Overflow questions to be chosen as well.
- Close as "off-topic, belongs on Stack Overflow" here, but also allow people to optionally specify a dupe target while voting to close. End result is a migration, followed by immediate closure as a duplicate of whatever question was chosen.
Based on your comments, you're looking for the former, but I'll mention both:
Allow cross-site duplicate targets when voting to close as duplicate
There might be something I'm not thinking of, but I don't have any ideological or philosophical objections to this. It seems like something that might cause a lot of strain on the backend, but I'm not a developer so that's their problem to solve or say it can't be done.
It has come up a few times on Meta Stack Overflow, with no outcome:
Allow users to migrate a question and close it as a duplicate after migration
I'm against this. Each community knows its site best: which questions should be closed, which should be open, etc. They should be the ones to decide if a new question is a duplicate of an existing question, not people on a different site who have no inherent interest in the other site.
Or to put it another way, let's say I have 10k rep here on Programmers, allowing me to vote to close, but only 1k rep on Stack Overflow. Allowing dupe closing during migration would allow me to close questions on Stack Overflow even though I'm 1,000 rep short of the privilege.
If you think a question is a duplicate of something asked on Stack Overflow, there are three options:
- If you can vote to close on Stack Overflow, vote to migrate, and keep an eye on it so you can vote to close when it gets there
- If you can't vote to close on Stack Overflow, vote to migrate and let SO users deal with it when it gets there
- Leave a comment pointing to the duplicate question on Stack Overflow and vote to close as off-topic in place here.
I'm partial to the last option: knowingly closing a question twice—once on each site—is just pointless bureaucracy.