I am going to replace my roof. I have one layer of shingles down and was going to re-shingle on top of them. Someone told me that when you put new shingles over old ones, it shortens the life of the new shingles. Is this true? Also, is the cost between shingles that last 25 years and those that last 50 years (asphalt-fiberglass) worth the cost difference in roof replacement?