Specifically the 100A breaker in the subpanel is redundant. The 100A breaker
in the main panel is required, to provide overcurrent protection for the
feeder to the subpanel.

In his new building, he needs both: one whichever panel the feeder is taken
from (to protect the feeder), and one in the subpanel in the new building (to
provide a disconnect for that building).

Doug et al

There is an exception in the NEC for this. If the feed from the main
panel is from a 60 AMP or smaller breaker, a main in the sub panel is
not required. Also some AHJs have made the exception moot by other
requirements. The additional/new panel can be rated at larger then 60
Amps also. Mine is 125 amp. Gives me 20 spaces I think it is. This
makes the panel a 60 Amp though. My neutral and ground are separated
and I have a 10" ground bar also. Habitable structure or not, also
makes a difference.

Bob AZ