Copying schematic or sheet? If copying sheets use Copy/Paste from the navigator. First copy the source sheet then use paste, but be aware that if you have a sheet selected in the navigator the paste will overwrite this selected sheet (think MS Word and how it works). To avoid this after doing the copy select the 'Schematic' node in the tree and then do the paste. If you copied sheet 1 for example the copy will give you a sheet 1_1 which you can then rename.
You can't copy a schematic directly, you'd need to create a new schematic and then paste the required sheets from the original schematic using the navigator.
If you want multiple designs in the project then create a new 'Board' and add schematics and sheets under this. Doing so allows you to share blocks across the different boards. So you could instantiate a block called say 'Fred' into both Boards, yet only have one copy of Fred to modify.
Thanks, I'll forward this information to our engineers.