Duplicating a track copies its clips, blends, and Inspector properties. If the duplicated track is bound to a GameObject, the binding is reset to None.
There are many ways to duplicate tracks:
Select a track. Right-click an empty area in the Track list and select Duplicate from the context menu.
Select a track. Hold Command/Control and press D.
Select a track. Hold Command/Control and press C, for copy, then press V, for paste.
Right-click a track and either select Duplicate from the context menu or hold Command/Control and press D.