That is uncommon, but it does happen. Although normally they have different flight numbers.
I had two other examples just this month (COPA from MCO to PTY, and then PTY to LIM - there are each two flights within 15 minutes, and yes, they were boarding at two neighboring gates, into separate planes).
I guess some connections just draw more customers for a certain time of the day than can fit in one plane.
What is possible is that the flight was moved, and one of the two you see now is the old one (before the moving), and the other is the new one. If the planes are different types, the airline is busy for some hours moving all passenger to new seats, and they simple lock the old one as full during that time. Once they have rebooked everyone, the old one will be removed.