"A Song is Born" question

[M_aria]

Since the Song+Nation single had Ayumi Hamasaki and Keiko from Globe on the cover, did they do "A Song is Born" as a duet? Or did each of them a song on the single, however I remember seeing the track listing and it only had "A song is Born" on it (JP Ver.) The "I am..." CD doesn't list Keiko anywhere near the lyrics to "A Song is Born" nor in the credits in the back.
So if there is only ONE version of the song, why was the heck was Keiko thrown in with Ayu for the single?!