how can my client(drag source) know where the user drag end at?

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I have read microsoft's drag drop documents and Michael Dunn's <<WTL for MFC Programmers, Part X - Implementing a Drag and Drop Source>> and many other search online.

but I still have no idea where the drop end, any way to know the destination folder the user drop at?

why I want to know this? if the user drag the same file to the desktop twice, I want to tip the user the same file has already existed. not the system give the user the tip. what I thought is if I know where the drop happened, I(the drag source) can get the destination folder, so I can check whether the file exists. then everything is fun.

any tips will be appreciate. thanks in advance.