FWIW, I have booked multiple tickets even on the same flight from time to time and have not have any trouble (with the same CC and SM#). That said, I usually do it only in the day or two before departure, and generally with fully refundable fares, so DL is likely to want the revenue
I do, however, believe it is strictly against the CoC, so one likely has little recourse should DL decide to cancel one (or more) duplicate/conflicting reservations during their flight firming processes.