Fix #80

Author: chrisburr

root_pandas/readwrite.py

@@ -2,6 +2,7 @@
 """
 A module that extends pandas to support the ROOT data format.
 """
+from collections import Counter
 
 import numpy as np
 from numpy.lib.recfunctions import append_fields
@@ -366,6 +367,11 @@ def to_root(df, path, key='my_ttree', mode='w', store_index=True, *args, **kwarg
     else:
         raise ValueError('Unknown mode: {}. Must be "a" or "w".'.format(mode))
 
+    column_name_counts = Counter(df.columns)
+    if max(column_name_counts.values()) > 1:
+        raise ValueError('DataFrame contains duplicated column names: ' +
+                         ' '.join({k for k, v in column_name_counts.items() if v > 1}))
+
     from root_numpy import array2tree
     # We don't want to modify the user's DataFrame here, so we make a shallow copy
     df_ = df.copy(deep=False)