B = list(set(A))

In [172]: lst = ['aaa','bbb','ccc','aaa','ccc','zzz']

In [173]: from collections import Counter

In [174]: c = Counter(lst)

In [175]: c
Out[175]: Counter({'aaa': 2, 'bbb': 1, 'ccc': 2, 'zzz': 1})

In [176]: [x for x in c.keys() if c[x] > 1]
Out[176]: ['aaa', 'ccc']

In [177]: [x for x in lst if c[x] > 1]
Out[177]: ['aaa', 'ccc', 'aaa', 'ccc']

for_stack = lambda array: sorted(list(set([x for x in array if array.count(x) > 1])))
print(for_stack(['a', 'a', 'b', 'b', 'c']))

['a', 'b']

for_stack = lambda array: sorted([x for x in array if array.count(x) > 1])

['a', 'a', 'b', 'b']