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i've had a couple years experience roofing...
if the new layer is only the second one and the bottom one doesn't have significant portions of soft/brittle shingles... then i would usually say it was ok to just lay over top.
try to walk the roof with the contractor beforehand and point out any worrisome areas. it's worth it to patch the old layer up a bit if there is serious danger of the old stuff coming loose underneath.
it's common practice though, for most cases i wouldn't give it much worry.
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If you will not fight when your victory will be sure and not too costly, you may come to the moment when you will have to fight with all the odds against you and only a precarious chance for survival. There may even be a worse case. You may have to fight when there is no hope of victory, because it is better to perish than to live as slaves.
~ Winston Churchill
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