ValueError: cannot reindex from a duplicate axis

import pandas as pd

df = pd.read_csv('data.csv', header=None, names = ['A','B','C'])

print(df)

    A   B   C
0   1   2   3
1   4   5   6
2   7   8   9
3  10  11  12

df['D'] = pd.np.nan # this creates an empty series
                    # and appends to the right

print(df)

    A   B   C   D
0   1   2   3 NaN
1   4   5   6 NaN
2   7   8   9 NaN
3  10  11  12 NaN

df = df[['D','A','B','C']] # rearrange as you like

print(df)

    D   A   B   C
0 NaN   1   2   3
1 NaN   4   5   6
2 NaN   7   8   9
3 NaN  10  11  12

df['new'] = df.index

colnames = df.columns.tolist()

colnames = colnames[-1:] + colnames[:-1]

df = df[colnames]

df = YourDataFrame
col = "Your Column You Want To Move To The Start Of YourDataFrame"
df = pd.concat([df[col],df.drop(col,axis=1)], axis=1)