Provide a helpful diagnostic when a -D flag starts with a redundant -D

Xcode (and possibly other IDEs) provide a UI over -D flags where you provide the flag name and -D is prepended for you. Users might reasonably add a redundant -D themselves, leading to the invalid flag `-D-DFOO` getting passed into the compiler. The driver could detect this and raise a specific error message alerting the user to their mistake.