it's using tape like pitch shifting
- shift the pitch down using tape and you get a wider spread of the harmonics in the vocal part
- shift the pitch up using tape and you get a narrower spread of the harmonics (crowding in ) in the vocal part
this is an old-school technique for using the same vocalist for lead and harmony vocals.
taken to an extreme here by applying to only single words/melodic pitches
adt (aka flanging in the traditional Beatles/George Martin sense)
also some multi-band gating from the sound of it (similar effect to autotune but no pitch correction)