To duplicate a limb, select any one of it's layers and click the Duplicate button. This will bring up a popup window that prompts you to change the first part of the layer name, with the idea that if you had named the first limb set Right you could now name the duplicate limb Left. Custom limbs can be duplicated just like default limbs, and the limb layer will be an exact copy of the one being duplicated, including any animation.
Select a Limber layer, click Copy, then select a layer from a different limb set and click Paste. The pasting process works by deleting the limb layer of the destination set and replacing it with a full blown copy of the copied limb layer, then updating properties so as to link it to the destination limb’s controllers. If you've added Effects to your limb, like Roughen Edges or Turbulent Displace, those will be copied over too.
The Limber effect properties (Lengths, Sizes, Colors etc) on the end controller are also pasted (except Clockwise and those in the FK Group). If you hold Alt whilst clicking Paste, the limb layer is pasted without changing those properties. This is often what you want when copying and pasting custom limb layers, eg. if you’ve already set the size and color of your destination limb.
When pasting, certain layer properties such as markers, masks, opacity, switches and effects on the destination limb layer, may be lost, replaced or reset. Keyframed properties are copied using the value at the current time. If your destination limb has keyframes on Limber properties affected by the Paste operation, the script makes a new keyframe for them at the current time.