How do you find the duplicate value in python code example

Example 1: duplicate in list python

a = [1,2,3,2,1,5,6,5,5,5]

import collections
print([item for item, count in collections.Counter(a).items() if count > 1])

## [1, 2, 5]

Example 2: python check for duplicate

def checkDuplicate(user):
    if len(set(user)) < len(user):
        return True
    return False