How do I find the duplicates in a list and create another list with them?

Question

How can I find the duplicates in a Python list and create another list of the duplicates? The list only contains integers.

Answer 1

I guess the most effective way to find duplicates in a list is:

from collections import Counter

def duplicates(values):
    dups = Counter(values) - Counter(set(values))
    return list(dups.keys())

print(duplicates([1,2,3,6,5,2]))

It uses Counter once on all the elements, and then on all unique elements. Subtracting the first one with the second will leave out the duplicates only.