[link to original on tumblr]
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First: the last lines of the first halves of the verses of "Cabin Essence" are doubled with a flute:
Light the lamp and fire mellowAnd
Cabin essence timely hello
Welcomes the time for a change
I want to watch you, windblown, facingIt's present in both versions - the 1966 one and the 2004 one. (I took the lyrics from the liner notes of the 2004 version, but I've changed the line breaks so it's easier to see how it's articulated.)
Waves of wheat for your embracing
Folk sing a song of the grange
I don't really have anything to say about the vocal/instrumental doubling by itself, but I found it interesting that Wilson re-uses this effect. It's also present (with viola) during the first verse of "I'm Waiting for the Day" from Pet Sounds.