When is doing a comp shingle overlay a good idea?

Just trying to learn more about the topic of comp shingle overlays (installing a second layer of comp shingles on top of the first without doing a tear off). Is it ever a good idea? If so, under what circumstances is it a good option to consider? What does one need to do to ensure a reliable overlay?

How long can you expect a “good” comp shingle to last? (if there is such a thing)