find duplicates in a df for each id(specific to each id check for duplicates) in pandas df

Question

df =
    id case value
1   1    a     1
2   1    b    34
3   1    c    56
4   1    c    56
5   1    d    34
6   2    a   546
7   2    a   546
8   2    b    67
9   2    c    23
10  3    a    65
11  3    b    44
12  3    c    23
13  3    c    23
14  4    a    87
15  4    b    34
16  4    c    87
17  4    c    87

expected output:indexes of duplicates [4,7,13,17]

In below df checking for duplicates for each id.any one please show me a path as i used duplicated method but im getting all the rows.please help.