Quoted from Kevin

Now you're just splitting hares.

Sorry, I can't help it. I'm an English major! And this question did honestly arise in my mind when I read the quote. Cheers, all.

Vocals = Paul on track 3 then Paul on track 4 - voila, it's plural!

Anyway, you can hear it's just Paul (or I can, anyway!) double tracked. There are lots of early tracks where John and Paul sing simultaneously and it doesn't sound like this. There is a very good example on the Help! album too - listen to Tell Me What You See and in particular the "look into these eyes now..." part - it's Paul AND John.