NY Post has an article by Michael Riedel on some reported "doubling" of some songs in Jersey Boys.  John Lloyd Young has posted a response detailing the "doubling" endeavors underway with Jersey Boys on his blog.  There is also some interesting analysis from The Stage Online (out of the UK).

The official response is:

"The authors of JERSEY BOYS
require any actor playing Frankie Valli (not specifically John Lloyd
Young) and other featured actors in the show to have a second voice
accompany them at certain moments during the show to recreate the
authentic sound of Frankie Valli and the Four Seasons.

The
multiple voice technique was pioneered on the original Four Seasons
records, and is talked about and explained in the show as part of the
story of JERSEY BOYS!

Everything in JERSEY BOYS is sung live. Everyone who appears to be singing on-stage is singing live. There are no tapes or click tracks. Accompanying vocals are performed by members of the cast singing on or off stage."

My own personal opinion is that if it was good enough for Frankie Valli & The 4 Seasons, then it is good enough for Jersey Boys.   And I don’t think it takes away at all from anyone’s personal performance.  Everyone is doing a fabulous job and giving 100% effort show after show.