I just don't think that this works. If you really believe that duplication of material on other websites is a problem, we will need a way to link to the duplicate on the other website. The SE software doesn't work this way right now.
Hmmm... I kinda liked that question. Seemed to fit the "best/worst UI" category. Didn't really understand the edit though.
If a question is actually better suited for another site, then it should probably be closed here. But there's an argument to be made in favor of asking similar questions to different audiences.
Should we be close duplicates of questions that came from other SE sites?
No, we shouldn't.
From my perspective, the existence of MathOverflow has nothing to do with anything here.
In fact, life should go on as if it doesn't exist because this is a totally separate site.
While MathOverflow is a non-SE site, we should have the same behavior with other SE sites.
We must see if the question could fit on our site, not if it could fit on other sites... Which brings us to:
Should the question be closed?
Yes, it needs improvement...
Let me quote the about page:
We are a site for expert programmers interested in subjective discussions on software development.
I'm sure bad website designs can be learned from and might fit in our domain,
but the question is stated similar to requesting people for their desktop wallpapers...
So I see it right to close this question for off-topic reasons, but it's not a duplicate in my eyes.