count the duplicates in a list in python

1some_list=['a','b','c','b','d','m','n','n']
2
3 my_list=sorted(some_list)
4 
5duplicates=[]
6for i in my_list:
7     if my_list.count(i)>1:
8         if i not in duplicates:
9             duplicates.append(i)
10 
11print(duplicates)