What does `ValueError: cannot reindex from a duplicate axis` mean?

I am getting a ValueError: cannot reindex from a duplicate axis when I am trying to set an index to a certain value. I tried to reproduce this with a simple example, but I could not do it.

Here is my session inside of ipdb trace. I have a DataFrame with string index, and integer columns, float values. However when I try to create sum index for sum of all columns I am getting ValueError: cannot reindex from a duplicate axis error. I created a small DataFrame with the same characteristics, but was not able to reproduce the problem, what could I be missing?

I don’t really understand what ValueError: cannot reindex from a duplicate axismeans, what does this error message mean? Maybe this will help me diagnose the problem, and this is most answerable part of my question.

ipdb> type(affinity_matrix)
<class 'pandas.core.frame.DataFrame'>
ipdb> affinity_matrix.shape
(333, 10)
ipdb> affinity_matrix.columns
Int64Index([9315684, 9315597, 9316591, 9320520, 9321163, 9320615, 9321187, 9319487, 9319467, 9320484], dtype='int64')
ipdb> affinity_matrix.index
Index([u'001', u'002', u'003', u'004', u'005', u'008', u'009', u'010', u'011', u'014', u'015', u'016', u'018', u'020', u'021', u'022', u'024', u'025', u'026', u'027', u'028', u'029', u'030', u'032', u'033', u'034', u'035', u'036', u'039', u'040', u'041', u'042', u'043', u'044', u'045', u'047', u'047', u'048', u'050', u'053', u'054', u'055', u'056', u'057', u'058', u'059', u'060', u'061', u'062', u'063', u'065', u'067', u'068', u'069', u'070', u'071', u'072', u'073', u'074', u'075', u'076', u'077', u'078', u'080', u'082', u'083', u'084', u'085', u'086', u'089', u'090', u'091', u'092', u'093', u'094', u'095', u'096', u'097', u'098', u'100', u'101', u'103', u'104', u'105', u'106', u'107', u'108', u'109', u'110', u'111', u'112', u'113', u'114', u'115', u'116', u'117', u'118', u'119', u'121', u'122', ...], dtype='object')

ipdb> affinity_matrix.values.dtype
dtype('float64')
ipdb> 'sums' in affinity_matrix.index
False

Here is the error:

ipdb> affinity_matrix.loc['sums'] = affinity_matrix.sum(axis=0)
*** ValueError: cannot reindex from a duplicate axis

I tried to reproduce this with a simple example, but I failed

In [32]: import pandas as pd

In [33]: import numpy as np

In [34]: a = np.arange(35).reshape(5,7)

In [35]: df = pd.DataFrame(a, ['x', 'y', 'u', 'z', 'w'], range(10, 17))

In [36]: df.values.dtype
Out[36]: dtype('int64')

In [37]: df.loc['sums'] = df.sum(axis=0)

In [38]: df
Out[38]: 
      10  11  12  13  14  15   16
x      0   1   2   3   4   5    6
y      7   8   9  10  11  12   13
u     14  15  16  17  18  19   20
z     21  22  23  24  25  26   27
w     28  29  30  31  32  33   34
sums  70  75  80  85  90  95  100

10 Answers

This error usually rises when you join / assign to a column when the index has duplicate values. Since you are assigning to a row, I suspect that there is a duplicate value in affinity_matrix.columns, perhaps not shown in your question.

As others have said, you’ve probably got duplicate values in your original index. To find them do this:

df[df.index.duplicated()]

Indices with duplicate values often arise if you create a DataFrame by concatenating other DataFrames. IF you don’t care about preserving the values of your index, and you want them to be unique values, when you concatenate the the data, set ignore_index=True.

Alternatively, to overwrite your current index with a new one, instead of using df.reindex(), set:

df.index = new_index

For people who are still struggling with this error, it can also happen if you accidentally create a duplicate column with the same name. Remove duplicate columns like so:

df = df.loc[:,~df.columns.duplicated()]

Simply skip the error using .values at the end.

affinity_matrix.loc['sums'] = affinity_matrix.sum(axis=0).values

I came across this error today when I wanted to add a new column like this

df_temp['REMARK_TYPE'] = df.REMARK.apply(lambda v: 1 if str(v)!='nan' else 0)

I wanted to process the REMARK column of df_temp to return 1 or 0. However I typed wrong variable with df. And it returned error like this:

----> 1 df_temp['REMARK_TYPE'] = df.REMARK.apply(lambda v: 1 if str(v)!='nan' else 0)

/usr/lib64/python2.7/site-packages/pandas/core/frame.pyc in __setitem__(self, key, value)
   2417         else:
   2418             # set column
-> 2419             self._set_item(key, value)
   2420 
   2421     def _setitem_slice(self, key, value):

/usr/lib64/python2.7/site-packages/pandas/core/frame.pyc in _set_item(self, key, value)
   2483 
   2484         self._ensure_valid_index(value)
-> 2485         value = self._sanitize_column(key, value)
   2486         NDFrame._set_item(self, key, value)
   2487 

/usr/lib64/python2.7/site-packages/pandas/core/frame.pyc in _sanitize_column(self, key, value, broadcast)
   2633 
   2634         if isinstance(value, Series):
-> 2635             value = reindexer(value)
   2636 
   2637         elif isinstance(value, DataFrame):

/usr/lib64/python2.7/site-packages/pandas/core/frame.pyc in reindexer(value)
   2625                     # duplicate axis
   2626                     if not value.index.is_unique:
-> 2627                         raise e
   2628 
   2629                     # other

ValueError: cannot reindex from a duplicate axis

As you can see it, the right code should be

df_temp['REMARK_TYPE'] = df_temp.REMARK.apply(lambda v: 1 if str(v)!='nan' else 0)

Because df and df_temp have a different number of rows. So it returned ValueError: cannot reindex from a duplicate axis.

Hope you can understand it and my answer can help other people to debug their code.

In my case, this error popped up not because of duplicate values, but because I attempted to join a shorter Series to a Dataframe: both had the same index, but the Series had fewer rows (missing the top few). The following worked for my purposes:

df.head()
                          SensA
date                           
2018-04-03 13:54:47.274   -0.45
2018-04-03 13:55:46.484   -0.42
2018-04-03 13:56:56.235   -0.37
2018-04-03 13:57:57.207   -0.34
2018-04-03 13:59:34.636   -0.33

series.head()
date
2018-04-03 14:09:36.577    62.2
2018-04-03 14:10:28.138    63.5
2018-04-03 14:11:27.400    63.1
2018-04-03 14:12:39.623    62.6
2018-04-03 14:13:27.310    62.5
Name: SensA_rrT, dtype: float64

df = series.to_frame().combine_first(df)

df.head(10)
                          SensA  SensA_rrT
date                           
2018-04-03 13:54:47.274   -0.45        NaN
2018-04-03 13:55:46.484   -0.42        NaN
2018-04-03 13:56:56.235   -0.37        NaN
2018-04-03 13:57:57.207   -0.34        NaN
2018-04-03 13:59:34.636   -0.33        NaN
2018-04-03 14:00:34.565   -0.33        NaN
2018-04-03 14:01:19.994   -0.37        NaN
2018-04-03 14:02:29.636   -0.34        NaN
2018-04-03 14:03:31.599   -0.32        NaN
2018-04-03 14:04:30.779   -0.33        NaN
2018-04-03 14:05:31.733   -0.35        NaN
2018-04-03 14:06:33.290   -0.38        NaN
2018-04-03 14:07:37.459   -0.39        NaN
2018-04-03 14:08:36.361   -0.36        NaN
2018-04-03 14:09:36.577   -0.37       62.2

Simple Fix that Worked for Me

Run df.reset_index(inplace=True) before grouping.

Thank you to this github comment for the solution.

I wasted couple of hours on the same issue. In my case, I had to reset_index() of a dataframe before using apply function. Before merging, or looking up from another indexed dataset, you need to reset the index as 1 dataset can have only 1 Index.

I got this error when I tried adding a column from a different table. Indeed I got duplicate index values along the way. But it turned out I was just doing it wrong: I actually needed to df.join the other table.

This pointer might help someone in a similar situation.

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