So, yeah. The key phrase is "from another SE site". Today I found this question on Chemistry.SE and I think that it quite close to this answered(!) question on Physics.SE. I'm not yet convinced that this two questions are duplicates of each other, but for the sake of argument lets us assume that they are. In fact, this is not the first time I came across a question which is already answered at Physics.SE.
I couldn't vote to close the question on Chemistry.SE as a duplicate since it requires the duplicate to be on Chemistry.SE and not somewhere else. What would be the best way then of handling such cases?