For example, this question:
It already has an answer on StackOverflow as in the comments but marking as duplicate per the close reasons is not an option. The duplicate must exist here. What should we do about these?
If the question is on-topic here, you can flag the off-site question for migration. You can also answer the on-topic question here; if a migration eventually occurs, then we can resolve the duplication or merge the answers as needed.
If the question is off-topic here, flag it or vote for closure here.
What's wrong with just using an answer to link to the offsite question? That might be the problem with this particular Stackexchange site...
You're over-complicating and adulterating a very simple and welcoming set of rules.