During the plagues of blood and boils there is a collaboration between Moshe and Aharon, and there is some repetition in the command and fulfillment. It seems like these two plagues are two of the signs that God gave to Moshe - the blood and the leprosy. Indeed, these two plagues introduce new groupings of plagues, allowing for the possibility that there were twelve plagues rather than ten.

The Ten Plagues
Courtesy of the Virtual Beit Midrash, Yeshivat Har Etzion