Even a porch should never have more than two layers.
And an architectural shingle will be considered two layers.
In the past, we would use a three tab - IKO calls them Marathons, not sure what the GAF equivalent is. That would allow a second layer without removing. .
Grace I&W would need a layer of shingles on the top. If the roof is shallow pitch, that is what will fail. It would need a lot of heat to stick flat and not curl.
Did the roofer give you a materials list? They may be looking at the job as a total... and that could reduce some waste.
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