Relevant old discussions:
It seems that closing as duplicate as the question has an answer on MO is not currently a valid close reason. Coming across this question which has several fine answers on MO (linked to in the comments), I wonder why.
Relevant old discussions:
It seems that closing as duplicate as the question has an answer on MO is not currently a valid close reason. Coming across this question which has several fine answers on MO (linked to in the comments), I wonder why.
isn't fully part of the network anymore
Not "anymore"; it never was. MO 1.0 (based on SE 1.0 platform) was separately administered. The current version is operated by SE, and takes advantage of network-wide support functions like DMCA compliance, but is owned by MathOverflow, Inc. See Who owns MathOverflow?
I wonder why
The SE platform has no built-in support for cross-site duplicate closure. It just wasn't designed with the MO/MathSE situation in mind. So, when circumstances warrant, I use custom closing reason: "this question has been asked and answered on MathOverflow". Yes, custom reasons are awkwardly nested under off-topic, but that's just another small limitation of the software.
And when do circumstances warrant this? In my opinion: when both of the following hold.
Otherwise, I will consider posting another answer on Math.SE: something with a more concrete example, a more detailed explanation, etc. And with a link to MO answer, of course.
(I offer no opinion on the particular question mentioned.)