My result is right and you say it's wrong

I would like to give some feedback about ...

From: http://www.checkio.org/mission/common-words/solve/

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Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/47.0.2526.106 Safari/537.36

My code is one line long:

def checkio(first, second):
    return u','.join([c for c in first.split(',') if c in second.split(',')])

#These "asserts" using only for self-checking and not necessary for auto-testing
if __name__ == '__main__':
    assert checkio(u"hello,world", u"hello,earth") == "hello", "Hello"
    assert checkio(u"one,two,three", u"four,five,six") == "", "Too different"
    assert checkio(u"one,two,three", u"four,five,one,two,six,three") == "one,three,two", "1 2 3"

And the test says :

 Your result: "one,two,three"
Right result: "one,three,two"
Fail: checkio("one,two,three", "four,five,one,two,six,three")

Which is wrong, because my result is ok.