DataFrame.melt fails if there are duplicate value_vars
DataFrame.columns.get_indexer fails if there are duplicate column names. 

https://github.com/pandas-dev/pandas/blob/05552d354a430f17a45b5fda8a1fd417f4c60c2e/pandas/core/reshape/melt.py#L109

This is really unfortunate -- melting should handle such cases

id   value value
id1  2       3 

should go to

id1 2
id1 3 

