Sorry to disagree but going over a single layer of shingles with a second one is common, even encouraged in the industry. I am not a professional but there is LOTS of impartial advice available. Building codes allow "go overs". Warranties allow it. It saves money. Of course it matters that the roof under the existing shingles is in good shape and the underlayment is not badly damaged. The existing roof also needs to be prepared properly, curled up shingles cleaned up, etc. A professional can do this well.
Just because a layer of shingles over another didn't age well doesn't disqualify the practice. I had a roof with a single layer of "30 year" shingles that looked bad at 12 years and had to be replaced at 15 years. Poor quality shingles are a common industry problem, even when they are from a supposedly good manufacturer.
I think it is more important that you know the reputation of the roofer and that he has been around for a while. If such a roofer recommended a second layer, I think it would be fine.
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