what condition our the existing shingles? we know they are 10 years
old. are they losing granules? corners wearing? or l;ooking spiffy?

in some areas like phoenix shingles age faster than others.

if you are planning on selling anytime soon or if the shingles are
aging you may be better served by a whole new roof.

apparently whoever added the 2nd layer of shingles didnt replace the
chimney flashing, that probably means the vent flashing, drip edge etc
are all at the end of their life too.

thats why i say consider a new roof, once done its a asset to your home