Called before the duplicator copies the entity.
If you are looking for a way to make the duplicator spawn another entity when duplicated. ( For example, you duplicate a "prop_physics", but you want the duplicator to spawn "prop_physics_my" ), you should add prop_physics.ClassOverride = "prop_physics_my". The duplication table should be also stored on that prop_physics, not on prop_physics_my.
Example on how to store values for duplicator, and then restore them afterwards