When you duplicate a limb layer using After Effects’ native Edit>Duplicate command, the new copy is effectively orphaned from the limb layer set. It is still linked to the same controllers, but Limber's functions will no longer be able to do anything with this orphaned layer. It's not part of the set of limber layers.
Sometimes you might need to use orphaned limb layers, and it can be useful to be able to copy and paste them. When you use Limber’s Copy button, Limber judges which limb layer you want to copy. If you have a limb layer selected, that is the layer that will copy, even if it’s an orphaned layer. But, if you have any other layer selected, e.g. a controller, it will copy the style of the original, non-orphaned limb layer. Similarly, when you paste a limb, it will only paste to an orphaned limb layer if you have that layer selected. If you have a controller selected, it will paste to the non-orphaned limb layer.
Orphaned limb layers are especially useful for matting. You can also use them when you need the upper and lower parts of a limb on separate layers, eg. so one can appear to be behind the body whilst another is in front of it.
You cannot use Limber's Replace or Duplicate buttons with orphaned limb layers. Trying to do so will result in the non-orphaned layer being replaced or duplicated instead.