Multiple layers of roofing shingles?
It was fairly common years ago, but even then there was a right way and a wrong way. The right way was to nest the new layer into the existing layer such that there's no structural span happening--each new shingle is fully supported. When that wasn't done the new layer was sure to fail prematurely. I watched it happen across the alley--a brand new roof came completely apart after about 8 years.

In my area the practice became outlawed some years ago (and manufacturers began to specifically reject warranty claims when it was done). It's rare to come across lingering roofs with multiple layers aside from the odd neglected outbuilding, for example.