This is the closest thing I could find in a quick skim thru the FAQ:
Q: Can Darth Bane use Master of the Force 2 to invoke Sith Rage twice for +20 damage on all attacks on that turn?
A: No. Stacking rules apply normally. Once you have Sith Rage you have it and having a second copy does not increase the damage. However, technically it can be invoked twice. It might be useful to do so if the first use is canceled with Force Absorb for example.
And from the glossary definition of "stacking":
Unless otherwise specified, no effect produced by a Force power, special ability,
commander effect, or cover stacks with itself.
Found the WOTC link:
http://forums.gleemax.com/showthread.php?t=1113967&highlight=knight