There was an interview with Stevie during the Rock A Little tour (national radio broadcast) where she talked about this song. Stevie said she loved Kenny but he was so strict in the studio and said "the whip would come down."
Its not unusual to not give another artist credit. Stevie never gave Prince credit on Stand Back. And no, I don't believe her story that Prince told her not to do it. But even if that was true, you still would give someone credit for playing critical parts of your first single from a new album.
However, to not give someone credit for singing a duet is pretty odd. Stevie does not just sing back up, she sings full verses.
As someone correctly pointed out, the Mac opened for Loggins and Messina in 1975 so that is when they got to know eachother well.
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 My heart will rise up with the morning sun and the hurt I feel will simply melt away
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