I would like to duplicate photos so I can keep originals and make amendments to a second copy. How do I do this?
As Mike says, in iPhoto, the software automatically keeps the original separate and creates a new file for edits and manipulations. This is true of many graphic software apps (e.g., GraphicConverter). Or you could do it manually by using Command+D after you have selected the files that you wish to duplicate.
Check free photo manager for MAC
By default you can create a duplicate of a file by simply pressing cmd + D.
If you are using iPhoto '11 to manage and edit your pictures you can simply go ahead and do so. It automatically saves a backup and you can find a button that says "Revert to original" at the bottom of the edit menu.
There's also an explicit duplicate action on the right click menu if you'd like to see them both in your library at the same time.