exercise: Find duplicates

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

visto = []

duplicates = []

for value in some_list:
    if value in visto:
        if value not in duplicates:
            duplicates.append(value)
            
    else:
        visto.append(value)
print(duplicates)