{char:text.count(char) for char in text}
import timeit
ex1 = """data = [1, 2, 2, 3, 3, 3, 4, 4, 4, 5]
res = [k for k, v in Counter(data).items() if v > 1]"""
ex2 = """data = [1, 2, 2, 3, 3, 3, 4, 4, 4, 5]
res = [i for i in data if data.count(i)>1]"""
if __name__ == '__main__':
print('ex1', timeit.timeit(ex1, setup='from collections import Counter'))
print('ex2', timeit.timeit(ex2))