Ditto

JARM introduces a way to duplicate your items, through the use of the Ditto.

To use the Ditto, simply hold it and right-click.
For versions 1.X: It will turn into whatever item is in the first slot of your hotbar, granted that item is not part of a stack.
For versions 2.X: It will turn into whatever item is to the right of it in your hotbar, granted that item is not part of a stack.

You can also specify which items the Ditto cannot turn into through the config file. This is to prevent people from duping a backpack full of Ditto's and all that.