This is an old question I know, but it's what I found when searching for a solution to this problem, so I think it's a good place to put the solution for fellow searchers:
- Drag a box around around the sheet you want to duplicate, using the
left mouse button.
- Release the left mouse button and press the right
to see a menu of things you can do. Select "Save Block".
- Then click on the little clipboard (paste) icon which will be somewhere around the edge of your schematic editing area (it's near the top left for me).
- Select where the new block will go.
This will paste a copy of your block, but it will refer to a new sheet based on the same filename as the original. If you annotate parts you'll see that the parts in the different sheets have different labels, even though they are sourced from the same schematic file.