Pandas df return indices of duplicated elements as a list

I’d like to have the indexes of duplicated column elements as a list. So far, the way I found is

test = ['a', 'a', 'b', 'c', 'b']
testdf = pd.DataFrame(test, columns=['test'])
np.asarray(np.where(list(testdf['test'].duplicated()))).tolist()[0]
# [1, 4]

Which seems ridiculously convoluted.

Any better way?

>Solution :

you can use .duplicated() with .tolist()

testdf.index[testdf.test.duplicated()].tolist()