If it is off-topic for Security.SE, I would suggest migrating to where it is on-topic and let them deal with it. If the question has already been answered on their site and they close it as a duplicate, that's fine. In other words, this is the "Migrate to SU, let them deal with it" option.
I don't understand why the fact that SU is likely to close it as a dup would make us shy away from migrating the question to the site where it belongs. If it is off-topic for us, it is off-topic. And if the question has already been answered there before, it has already been answered, and there's not really any need to repeat that.
If it is on-topic for Security.SE, I would suggest leaving it here, even if there is some overlap with another question on another community. This community often has a different perspective and provides a different sort of answer.
Example: If asking about conceptually how data recovery works and under what circumstances it might be feasible, then that's the sort of question that our community can probably give great answers to. If asking for a step-by-step recipe of how to do it, that might be the sort of thing that ServerFault will have better answers for. So, our judgement about whether it is on-topic or off-topic might depend upon the details of exactly what is being asked.
If it is in a grey area, Rory Alsop's suggestion makes sense to me: figure out if it is high-value to us or not; if it is high-value, leave it here; if it is not high-value to us, migrate to SU and let them deal with it, or close it.