cannot reindex duplicate axis - python-2.7

Question

I am trying to merge multiple csv files in a folder.
They look like this (there are more than two df's in actuality):
df1
LCC acres
2 10
3 20
4 40
5 5
df2
LCC acres_2
2 4
3 2
4 40
5 6
6 7
I want to put all the dataframes into one list, and then merge them with reduce. To do this they need to have the same index.
I am trying this code:
combined = []
reindex = [2,3,4,5,6]
folder = r'C:\path_to_files'
for f in os.listdir(folder):
#read each file
df = pd.read_csv(os.path.join(folder,f))
#check for duplicates - returns empty lists
print df[df.index.duplicated()]
#reindex
df.set_index([df.columns[0]], inplace=True)
df=df.reindex(reindex, fill_value=0)
#append
combined.append(df)
#merge on 'LCC' column
final = reduce(lambda left, right: pd.merge(left, right, on=['LCC'], how='outer'), combined)
but this still returns:
Traceback (most recent call last):
File "<ipython-input-31-45f925f6d48d>", line 9, in <module>
df=df.reindex(reindex, fill_value=0)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\frame.py", line 2741, in reindex
**kwargs)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\generic.py", line 2229, in reindex
fill_value, copy).__finalize__(self)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\frame.py", line 2687, in _reindex_axes
fill_value, limit, tolerance)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\frame.py", line 2698, in _reindex_index
allow_dups=False)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\generic.py", line 2341, in _reindex_with_indexers
copy=copy)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\core\internals.py", line 3586, in reindex_indexer
self.axes[axis]._can_reindex(indexer)
File "C:\Users\spotter\AppData\Local\Continuum\Anaconda2_2\lib\site-packages\pandas\indexes\base.py", line 2293, in _can_reindex
raise ValueError("cannot reindex from a duplicate axis")
ValueError: cannot reindex from a duplicate axis

Answer 1

There is problem you need check duplicates of index after setting first column to index.
#set index by first column
df.set_index([df.columns[0]], inplace=True)
#check for duplicates - returns NO empty lists
print df[df.index.duplicated()]
#reindex
df=df.reindex(reindex, fill_value=0)
Or check duplicates in first column instead index, also parameter keep=False return all duplicates (if necessary):
#check duplicates in first column
print df[df.iloc[:, 0].duplicated(keep=False)]
#set index + reindex
df.set_index([df.columns[0]], inplace=True)
df=df.reindex(reindex, fill_value=0)