Its not too uncommon to see that. Before the merger, HP had two SAN-PHX flights that left 10 minutes apart (you know how confused people were when they left out of neighbor flights - I was one of the few who didn't get it wrong when I did the route). Also, NW did two DTW-NRT about 30 minutes apart.

Sometimes they add the extra flight b/c the market demands it during a certain time frame. Others times, its the only time the extra plane is available to meet the market demands.