Quite a few of the Marvin and Tammi duets on the UNITED and YOU'RE ALL I NEED albums were originally Tammi solos that Marvin later added his vocals to. So that started even before Tammi's collapse on stage in 1967.
Not sure of the timeline but by early 1968, Tammi may have already been operated on at least once. Her illness is referenced in the liner notes to the YOU'RE ALL I NEED album, which was released in August 1968. So by that point, there might have been more of a need for Marvin to overdub on Tammi's older recordings.
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