Select the artboard title and cmd/D
This is a good answer, but I still had to poke around for where the Artboard title is:
You can double click the Artboard title just above the actual Artboard. Renaming them really helped me when linking together. Do you know if there is a way of copying the linking properties of a button as well?
Best way I've found —
In artboard mode (Shift + O), hold Shift + Opt + Command and drag whichever artboard you want to duplicate.
Everything on the artboard is duplicated, and the layer order is preserved (if you have things on different layers on that one artboard).
There is a keyboard or something to add "x" numer of artboard if I have already one done? I want duplicate but not one for one
Easy way, just go to your artboards (click in the grey next to your artboard) and hit right mouse btn and then duplicate
ANSWER for WINDOWS ///////
Select the Artboard (D) and press Crtl+Alt + drag the new artboard.