Purchasing a second seat because you need/want the extra empty seat is a different process than going online and making two bookings in your own name.

While OP's question is generic and thus a specific answer is not possible, most carriers catch duplicate bookings well before flight time and require the passenger to cancel one (or the carrier cancels one or both).

Even if not cancelled and even if one checked both reservations in, one person can only board once. Part of the process of closing out a flight is reconciling the number of BP's swiped against the number of people physically onboard. Thus, even if by some sleight of hand, both BP's were scanned, the count would be of.