There’s a recording, that is used for 2 separate versions of a track (one with a featured artist, one without). Both distinctly different recordings. It seems they were globbed together at some point, and I’m not sure how to separate them properly (and without mess) now.
From the release, you can see track #5 has no featured artist, track #13 does have a featured artist. Both use the same recording erroneously.
Could you listen to the linked preview clip to make sure it’s not just uncredited?
I don’t know how Christina Aguilera sounds like.
edit
Oh sorry, hearing these previews, I see she sings on track 12.
So indeed this recording should be split.
We first have to check edit history to see if it was created for the duet version and was later hijacked by the solo version. Or the opposite.
Apparently the recording was created for solo version but then multiple recording merges occured.
See: