This guide will tell you how to Duplicate & Delete Custom Loadouts in Modern Warfare 2.
You will need to open Options by pressing the left thumb stick and then clicking on Duplicate from the options menu on the right side of the screen. After that, assign a loadout position for duplication and it will duplicate your loadout to the custom loadout which you have selected from the list.
If you want to delete the loadout, you have to select it and then click on the Delete button from the options. After that, it will be no longer available on your list.