Delete unique elements from a list

Question

I faced some problem with solving the next problem:

We have a list of elements (integers), and we should return a list consisting of only the non-unique elements in

Answer 1

Just I used list Comprehensions.

def checkio(data):
    a=[i for i in data if data.count(i)>1]
    return a
print checkio([1,1,2,2,1,1,1,3,4,5,6,7,8])