You can simply assign the B
to the new column , Like -
df['D'] = df['B']
Example/Demo -
In [1]: import pandas as pd
In [2]: df = pd.DataFrame([['a.1','b.1','c.1'],['a.2','b.2','c.2'],['a.3','b.3','c.3']],columns=['A','B','C'])
In [3]: df
Out[3]:
A B C
0 a.1 b.1 c.1
1 a.2 b.2 c.2
2 a.3 b.3 c.3
In [4]: df['D'] = df['B'] #<---What you want.
In [5]: df
Out[5]:
A B C D
0 a.1 b.1 c.1 b.1
1 a.2 b.2 c.2 b.2
2 a.3 b.3 c.3 b.3
In [6]: df.loc[0,'D'] = 'd.1'
In [7]: df
Out[7]:
A B C D
0 a.1 b.1 c.1 d.1
1 a.2 b.2 c.2 b.2
2 a.3 b.3 c.3 b.3