OK, I will run afoul of some roofers here, but in my opinion one should never put shingles on top of shingles. Various govt housing agencies do not allow this at all, but it really isnt a good thing. The upper layer of shingles doesnt mount as well and can blow off. For instance, we had a large storm yesterday and the one home in the neighborhood that had overlayed their house had their shingles blow off. Plus it makes the roof heavy and can make it unatractive. Just pay the extra to have the old shingles stripped, the plywood inspected, and now shingles and tarpaper installed. You will be better off in the long run.