How to get a list of all indices of repeated elements in a numpy array?

Question

I\'m trying to get the index of all repeated elements in a numpy array, but the solution I found for the moment is REALLY inefficient for a large (>20000 elements) input

Answer 1

@gg349's solution packaged up into a function:

def better_np_unique(arr):
    sort_indexes = np.argsort(arr)
    arr = np.asarray(arr)[sort_indexes]
    vals, first_indexes, inverse, counts = np.unique(arr,
        return_index=True, return_inverse=True, return_counts=True)
    indexes = np.split(sort_indexes, first_indexes[1:])
    for x in indexes:
        x.sort()
    return vals, indexes, inverse, counts    

It's essentially the same as np.unique but returns all indices, not just the first indices.