Set message for duplicate value pandas

Question

i have df like this

   A   B 
0  1   ABC
1  2   XYX
2  1   RTC
3  3   fds
4  2   rtv
5  4   rtoc

and i want like this

    A   B     message
0  1   ABC     
1  2   XYX    
2  1   RTC     Duplicated 
3  3   fds     
4  2   rtv     Duplicated
5  4   rtoc

if Column A value is duplicated then set the message duplicated in second occurrence.

Answer 1

all duplicates:

Use numpy.where and duplicated:

df['message'] = np.where(df['A'].duplicated(), 'Duplicated', '')

output:

   A     B     message
0  1   ABC            
1  2   XYX            
2  1   RTC  Duplicated
3  3   fds            
4  2   rtv  Duplicated
5  4  rtoc            

only for the SECOND occurrence:

Use groupby+cumcount:

df['message'] = np.where(df.groupby('A').cumcount().eq(1), 'Duplicated', '')

example:

   A                  B     message
0  1   first occurrence            
1  2                XYX            
2  1  second occurrence  Duplicated
3  1   third occurrence            
4  3                fds            
5  2                rtv  Duplicated
6  4               rtoc