You can duplicate an item from anywhere you can edit an item.
Either in your item bank or the assessment builder, click the item you wish to duplicate, click edit, then scroll down to the item editor.
Next to the Save button on the right, there is a dropdown triangle that opens up the option to duplicate the item.
Once you've duplicated the item, you will return to the item original. If you would like to see your newly duplicated item, you can click on Item Bank and pick the topmost item, as most recently created items show up on top.
Feel free to edit your newly duplicated item as you see fit by clicking edit in the top right corner.