out of "order"

The words are out of order, so it didn't count it.. My code:

a= first.split(",")
b= second.split(",")
c = ""
for x in a:
    for y in b:
        if x == y:
            c = c + " " + y
return c.strip().replace(" ", ",")

My result: one,two,three the "correct" result: one,three,two

Come on now... :P

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

HTTP_USER_AGENT:

Mozilla/5.0 (Windows NT 10.0; WOW64; rv:41.0) Gecko/20100101 Firefox/41.0