what method to use for duplicate element in a list 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 duplicate list elements

def duplicate_list_elements(_list : list):
    duplicated_list = []

    for element in _list:
        duplicated_list.append(element)
        duplicated_list.append(element)

    return duplicated_list