PROBLEM
Graphical, layer based programs often allow for duplicating layers while holding alt. This allows users to duplicate an object or collection by dragging with alt and to drop it at the destination of choice. Currently, the closest one can get is to assign alt+Left mouse dragging to duplicate an object or collection, but this does not allow one to drag & drop the duplicate. Using hotkeys is another option, but in certain situations require to hit a hotkey to duplicate and drag & drop the duplicate in the collection of choice, which requires more inputs.
SOLUTION
Please allow modifier key + drag and drop to duplicate objects or collections in the outliner.