Delta indeed may notice this and cancel one or more of your bookings. From their policies (in a page intended for travel agents):
All duplicate bookings generated by a single GDS subscriber are
Multiple itineraries for any number of passengers with the same passenger name, whether identical itineraries or not
Reserving one or more seats on the same flight or different flights for the same time frame, regardless of the class of service or
format used to make the reservations
Additionally, creating a reservation where it is logically impossible
to be used on each segment created across one or more PNRs or GDSs is
And from their contract of carriage:
E) Duplicate, Fictitious and impossible/illogical bookings
Delta prohibits duplicate, impossible, or fictitious bookings, including but not limited to multiple conflicting itineraries for the same passenger on the same day or bookings with connections that depart before the arrival of the inbound flight. Delta reserves the right to cancel any such booking which has not been ticketed, and to cancel and refund any such booking which is ticketed at a refundable fare.
They do have systems to catch this, where it's physically impossible for you to take both flights. Anecdotal experience varies on how this is enforced, with some people saying it worked out and some saying that the airline canceled one or contacted them to make them choose which reservation to keep.