Does rend stack?
Does the Barbarian skill “Rend” stack? If not then it is waste of fury to cast it more again before the previous cast ended?
How should this skill be used?
No it won't stack on top of your own rend.
Two Barbarians can cast rend and they will stack, but casting rend again on your own will only refresh the current rend, not add a new DoT.
This is actually how majority of DoT's work in Diablo 3, they will all refresh themselves but not stack, unless two different characters cast the same spell.
The only exception I can think of is Magic Weapon - Venom for Wizards, it will stack on itself.