Many schools, such as George Mason University, offer an accelerated MA program for economics in 4 years. If the goal is to apply for a PhD program at a top institution, would it be better to go down this route or to double major in economics and mathematics? Would graduate admissions view candidates with these qualifications as equally qualified or is there one option that’s better than the other?
I don’t think it really matters that much. I think what department you are coming from and what experiences you have matter more. An undergraduate with just a BA who has written a rigorous senior thesis may be preferred to a student who did a coursework-only MA. I think all other things equal it would be great to take some graduate-level classes in economics, but you can do that even if your program doesn’t formally offer a BA/MA program.
I’d select an undergrad based on other factors - like what’s the best overall experience for you - but ask about the potential for taking graduate-level economics classes and getting research/scholarship experience with professors.