I am a long time MacOS and Photoshop user and much of my work is creating interface mockups. Using Command + Option + Drag to duplicate a pixel selection within a single layer is essential to my workflow.
The suggestions above do not tally with my experience. When I use Affinity Photo, Command + Drag will cut and move a selection (but not duplicate). The option and control modifiers only subtract and add to the selection respectively.
Maybe I'm missing something but it seems the only way to achieve what I need is to cut and paste a selection to a new layer, move that layer and then merge it down again - this is a really slow and clunky solution.
The thing is, using Command + Option + Drag to duplicate is a long-standing MacOS paradigm and my muscle memory is so ingrained that I think I will have to return to Photoshop to get my work done.