def checkio(a):
    #Your code here
    #It's main function. Don't remove this function
    #It's used for auto-testing and must return a result for check.
    source = set(a)
    source = dict.fromkeys(set(a), 0)


    for x, v in source.items():
        for y in a:
            if v >= 2:
                break
            if x == y:
                source[y] += 1
    for k, v in source.items():
        if v == 1:
            a.remove(k)

    #replace this for solution
    return a