Currently using the keyboard to copy and paste makes the copied object appear in the middle of the diagram, which is really not very useful, especially if you have a big diagram.
It would be great if the new object would appear on top off and offset from the original object. Failing that, either the centre of the visible window, or else wherever the mouse happens to be.
I agree that would make a lot sense.
In the meantime (on my platform), I just option-drag to copy. It is no loss here not to have a completely keyboard way, since I will need to be on the mouse to place my new node anyway.
UPDATE:
So Ctrl+C Ctrl+V on an object actually stacks downwards, which is fine.
However,
If you click anywhere in the window and then press Ctrl+V, Scapple pastes into the centre of the drawing!
This looks like a bug.