I've been asked this questions more times than I can count over the past seven years. The answer is pretty straightforward. You can do whatever you want. A better question would be SHOULD you install metal roofing over shingles and that question is a little more involved, so let's take a look.
In short, yes, metal roofing can be installed on top of shingles. However, there are a few variables that come in to play before installing your new metal roof.
What is the condition of your roof deck?
Have you had roof leaks before? How about large limbs or hail that may have left impact damage? If your roof deck has sustained water, fire or impact damage then you should remove the existing shingles and repair/replace the damaged roof deck areas before installing your new metal roof. Installing new roofing materials over a damaged roof deck will lead to larger and more expensive problems down the road. You can get an idea of the condition of your roof deck by accessing your attic space and inspecting the bottom side of the substrate.
What is the condition of your existing shingles?
If your current shingle roof has sustained significant damage or has begun curling then you should either remove it or install lathing boards before installing a new metal roof (assuming the roof deck is in good condition). Significantly damaged or curled shingles don't provide a planar surface for your metal roof to be installed on and may therefore, at best, dramatize oil-canning effects and profile through the metal roofing panel. At worst it could lead to the failure of your metal roofing system if it prevents proper laps, sealing, etc.
How many layers of shingles do you currently have?
Most building codes allow for two layers of roofing materials on your home. If you already have more than one layer of shingles then you would need to remove a layer (or both) before installing your new metal roof. While metal roofing is only about one-third to half the weight of asphalt shingles, you should still never install more than two layers of material on your roof and again, most local building codes won't allow it anyway.
Once you've figured out the answers to these questions there are a few more frequently asked questions regarding proper procedure for installing metal over shingles:
Should I install the metal directly on the shingles?
No. You should never install metal roofing directly on shingles without first installing a synthetic underlayment or a spacer like lathing boards or Retro-Ease Spacer. Metal roofing expands and contracts as it heats up and cools down (thermal expansion). If left to constantly move against the abrasive grit on asphalt shingles you may introduce a scenario where the primer on the back of your metal roofing panels is scratched off exposing the panel's substrate and creating an environment for rust to occur. Most coating manufactures won't cover scenarios like this under their paint warranty so be sure to use a barrier between your asphalt shingles and your metal roofing panels.
Is there any benefit to installing metal over shingles as opposed to tearing them off?
Yes! While if installed correctly a metal roof will be a lifetime roof with minimal maintenance, installing over existing shingles gives you one more layer of protection between the elements and the inside of your home. You'll also likely save a substantial amount of money on labor by avoiding a costly tear-off that prolongs the project's completion and can often lead to landscape and property damage.