Error tests

I would like to give some feedback about ... From: http://www.checkio.org/mission/common-words/solve/ HTTP_USER_AGENT: Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/46.0.2490.86 Safari/537.36

my solve:

def checkio(first, second):
    return ",".join([w1 for w1 in first.split(',') for w2 in second.split(',') if w1==w2])

Give error: Your result: "one,two,three" Right result: "one,three,two"