Finder on Yosemite misbehaves: doubled text, blank windows, lack of updates
You have to make a PRAM reset.

To do that restart your Mac, and BEFORE the "Bong" press ALL the following keys, until you hear the "Bong" again : cmd + alt + P + R
After your can let your Mac start normally.

If the problem persist, redo the manipulation but stay your finger on the key until you have hear the "Bong" 5 times.