If you're going to live in the house for the next 30+ years, I'd get a 30 year shingle AND have the existing shingles removed. If you're going to move soon, then just go over top.
There are Building Code issues. Make sure your roof can carry the WEIGHT of the 2 layers. The disadvantage is that the top layer may not lay very well on top of the bottom layer. If the bottom layer is EXTREMELY aged, I'd definitely remove it.
Wind is a problem in a lot of places. That's why I recommend a heavy shingle such as a 30 year shingle. You can always use a dab of black roofing caulk under the first few rows of shingles at the roof line and rake edges to help keem them down.
My best to ya and hope this helps.
Jay J -Moderator
PS: God Bless America
PPS: You'll get what you pay for ...