Accidentally bought a "butter basted" turkey. Can/should I still brine it?
Pre-basted turkey is indeed already brined. You can double check this by looking on the ingredient list of the turkey packaging and you should see a salt or vinegar solution listed.

You can still soak the bird if you would like, but either use no salt or a low-salt brine, like a 0.5-1% solution. In this case you will probably be pulling out some of the salt from the brine that's already on the turkey.

Here's a description from The Food Lab (a pretty rigorous source) on these types of turkeys:


  Self-basting birds have been injected with a salt and flavor solution
  to help keep them moist while cooking. They come out incredibly moist,
  almost wet, and can be cooked directly from the package with minimal
  pre-roast work required. They also tend to be dull and diluted in
  flavor.