When you are creating a skeleton, you often need to duplicate chains and then "mirror" them across the correct plane. With the Duplicate Symmetry command, you can do both at once.
You can select the plane across which you want the duplicated chain to be mirrored: XY, XZ, YZ. For example, say you've created a simple arm chain in the Top view. To duplicate and mirror it for the arm to be on the other side of the body, you would choose the command and then select the YZ plane.
On the top, original chain is duplicated across the YZ plane in the Top view.
On the bottom, original chain is duplicated across the XY plane in the Top view.
The Duplicate Symmetry command also duplicates constraints (such as position and up-vectors) and chain properties (such as rotation limits), which means that you can duplicate entire rigs for skeletons.
As well, the Duplicate Symmetry command works on any geometrical object or hierarchy, not just chain elements.
To duplicate and mirror a chain
Select the (unanimated) 2D or 3D chain you want to duplicate (middle-click to branch select or right-click for the whole hierarchy).
Choose Create 
Skeleton Duplicate Symmetry from the Animate toolbar or Edit Duplicate/Instantiate Duplicate Symmetry.
In the Duplicate Symmetry dialog box, select Duplicate Constraints to include existing constraints on the chain you're duplicating. For example, if there's an up-vector constraint on an arm or leg, you may want to use it on your new chain as well.
If the chain you want to duplicate is constrained to a null, for example, and you select only the chain and choose this command with Duplicate Constraints selected, the new symmetric chain is also constrained in position to the original null.
If you select both the chain and the null, the null is duplicated and the new symmetric chain is constrained to the new null.
Select Share Parent to automatically make the duplicate a child of the same parent as the original. If you don't select this option, the new chain will be a child of the scene root.
Select Freeze Negative Scaling to do just that on the duplicated chain.
If you deselect this option, the scaling for the duplicated chain is negated, switching its handedness. For example, if the original chain is right-handed, the duplicated chain will have a left-handed coordinate system. Note that negative scaling may not be compatible with game engines or motion capture data.
