In my previous post, I discussed the superb bridge on "Ask Me Why" and noted that, on the first section of each couplet, it seemed that either John's vocal was doubled or Paul's unison part blended in almost without a trace. On a separate website that runs these commentaries, one person took up this specific point and commented that "A Taste Of Honey" is the only entry on Please Please Me that features a "'trick duet.'" This term denotes a vocal that was recorded twice by the same person, but sounds as if two voices were at work. Thus, John and Paul were in fact singing in flawless unison on the part mentioned above.
It wasn't until 1967 that the more advanced and convenient technique for doubling, ADT (Automatic Double Tracking), was available to the Beatles. Here's an excerpt from the Revolver page on Wikipedia which explains the matter further:
“A key production technique used for the first time on this album was automatic double tracking (ADT), invented by EMI engineer Ken Townsend on 6 April 1966. This technique used two linked tape recorders to automatically create a doubled vocal track. The standard method was to double the vocal by singing the same piece twice onto a multitrack tape, a task Lennon particularly disliked. The Beatles were reportedly delighted with the invention, and used it extensively on Revolver. ADT quickly became a standard pop production technique, and led to related developments, including the artificial chorus effect.”