How to duplicate the same value in the list python code example

Example 1: count the duplicates in a list in python

some_list=['a','b','c','b','d','m','n','n']

 my_list=sorted(some_list)
 
duplicates=[]
for i in my_list:
     if my_list.count(i)>1:
         if i not in duplicates:
             duplicates.append(i)
 
print(duplicates)

Example 2: 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]