It would be very helpful if we could vote to close questions as exact duplicates of questions on MO.
edit to consolidate some ideas fleshed out in the comments on Thomas Owens's answer:
- Having the possibility to close a question as a duplicate of an MO question does not mean that all questions that are (partial or exact) duplicates have to or will be closed as such. In particular, I think that if the question is likely to generate new/different answers here than it did on MO, it's worth having here separately.
- Closing a question as an exact duplicate of an MO question would create a prominent link to the MO question so that anyone coming across the question here should readily be able to get to the MO question with the answers.
- It's only worth closing as an exact duplicate if the MO question provides a complete answer (or set of answers), so that any answers here would be duplicating those on MO (people spending time unnecessarily redoing the work of answering, people copy/pasting answers from MO, or people answering with links to MO).
- I'm not saying that a question posted on MO is off-topic here. In fact, I believe that any question (properly) posted on MO is inherently on-topic here.