Check for duplicates in a list

some_list = ['a', 'b', 'c', 'b', 'd', 'm', 'n', 'n']
seen = set()
duplicates = set()

for char in some_list: #loop through chars
    if char in seen: # if char is present in seen -> add to duplicates
        duplicates.add(char)
    seen.add(char) # otherwise if char is not present in seen -> add to seen

print(list(duplicates))