Roof update: remove existing or install on top of current roof?

[Edita D.]

Hey guys!

I received 4 bids to replace the roof, and I am really confused. Some contractors say there's MORE than 1 layer, some say there's only 1 layer of older roof.
First, Is it necessary to remove the existing roof or is it ok to install a new roof on top of existing one?
Second, If I decide to install the new roof on top of old one, am I taking a risk of continuance of wood rot if it DOES exist currently (in the wood sheathing)? We live in sunny San Diego, it doesn't rain here much.
Thank you,
Edita