Things might be different in warm climates, but here in the north where it snows, its entirely optional to remove the first layer on re-roofing unless the asphalt shingles are very deteriorated in places, making a smooth application difficult for the 2nd new layer. Most people just cover over (do check your warranty also). The reasons are added insulation/protection being the primary, and second the cost, not to mention the mess it creates. You might have noticed that roofing nails come in various lengths. Provided you can nail through into the plywood or boards, go for the 2nd layer and some even lay on a 3rd layer. Its not uncommon to see three layers on roofs in the Toronto area. If your roof shingles are badly deteriorated in some places, but not others, or curling, chances are your roof has a venting issue. Make sure its properly vented when doing the re-roofing, and personally, stick to the metal valleys. There's a lot of wear from the elements in that valley, and of course you know why shingles are speckled with with ceramic granules, right? Of course you do - it protects the asphalt from ultra-violet light and prolongs shingle life. This is why you don't want the branches of an overhanging tree limb wearing on your roof, not to mention the ice and snow that can form in the valley.
Oh, and if you do go for layer-removal, make sure you buy a magnet on a pole (as in broom pole) and sweep with care, because the roofers will leave nails behind, no matter how diligent they appear to be; and roofing nails can become a nasty projectile when the lawnmower blade connects. I guarantee you'll find them all the way out into the street.
If your roof has any 'belly' going on, don't just replace the sheet. Check the rafter also. You might need to marry on another stick of wood to bring the surface back to it's true slope or it could be damaged and need replacing altogether.
One last note on behalf of the spelling police - its 'ridiculous' with two i's. I keep seeing 'rediculous
' written here in this forum. For some reason it bothers me, and i can't explain why...