df:
col1     col2
 1        10
 1        20
 2        11
 3        33
 1        20
 1        10
 2        24
 3        21
 3        28

df
col1    col2
 1       20
 2       11
 3       33
 1       10
 2       24
 3       28

df['b_new'] = df.groupby('col1')['col2'].transform('last')