pandas - check for non unique values in dataframe groupby

Question

I have this simple dataframe df:

a,b
1,2
1,3
1,4
1,2
2,1
2,2
2,3
2,5
2,5

I would like to check whether there are duplicates in b with respect to each group in a. So far I did the following:

g = df.groupby('a')['b'].unique()

which returns:

a
1       [2, 3, 4]
2    [1, 2, 3, 5]

But what I would like to have is a list, for each group in a, with multiple occurrences in b. The expected output in this case would be:

a
1    [2]
2    [5]

Answer 1

g=df.groupby('a')['b'].value_counts()
g.where(g>1).dropna()

Answer 2

We can use duplicated

print(df[df.duplicated()].drop_duplicates())