TomAugspurger/pandas — pull request #2 by TomAugspurger (2014-04-09 15:33:17+00:00)
Title: API/BUG Raise ValueError when stacking nonunique levels

Should raise a ValueError when (un)stacking a DataFrame
on a nonunique level. Previous behavior was to raise
a KeyError (not deliberately). Closes #6729.