At the moment if I want to duplicate my selection content to the same layer I can do that by holding ctrl modifer and clickdrag the selection.
What is very practical (in photoshop) if you want to extend the edge of an secltion content while you are putting down the basic forms and outline. it works exactly the same as it does now using arrow keys with a selection. Now you move the content of the selection. Holding Ctrl disables any movement with arrows keys. Which I think it for consistency should not.
So basically if I have a selection, hold down the Ctrl and press left arrow. The content is moved one pixel to the left but the original is left in place = duplicate it. I tworks exactly as if I would hold Ctrl and click drag one pixel to the left but of course its a lot more convenient and quicker to just use arrow keys.
I hope this would be a very quick thing to implement as duplicate is already working and moving a selection is also working.
If anything is unclear just ask me and I send a image or a movie explaining it visually.