BiggerPockets

I am about to start a project on a house and my contractor says that instead of replacing the current roof he can do a "nail over" of new shingles on top of old.

have you ever done this? is there a problem with doing this?

As Jon says, it can be done and was once standard practice. But, here recently, every roofer I talk to says we need to tear off the old roof. I have been told every reason from city codes to the weight of the old shingles. I haven't had good luck with roofers thinking like me. I like the way yours thinks.
Don

Here, two layers are legal, but I'd never do it, nowadays you want (and its code in a lot of places) to use Ice & Water shield under the shingles in some areas, and that needs bare sheathing.

If you want to develop a good reputation, then don't do it. It will reduce the life of the new shingles and make it difficult to locate leaks for the most part. The roof is the front line of defense for the structure. It's not a place you ever want to cut corners.

In our state 2 layers is acceptable when it comes to the lender. Take note that it does void the warranty on the shingles. In order for the warranty to remain valid you must remove all old shingles down to the decking. And, it usually makes for a better looking roof as well. My vote would be to do a complete tear off and if you have to skimp to make your numbers work skimp on something that you are doing for your "ego" as opposed to making money on the deal.