I don't believe that has ever happened. An integral part of being declared a saint (canonized) in the Catholic Church is that the person is declared to be in Heaven, so that precludes still being alive on Earth. The Dominican priest (and professor) who inspired my username asserted that the Church waits for the subject to be "safely dead" before beginning the canonization process. He was joking, but only to an extent. Having the subjects story be complete is important, so that you would avoid a saint denying God's existence or the like in the twilight of their life.

None of this is to say that the Church always had a set procedure in place. The modern procedures were revised in 1983, and before that in 1917 and 1914. Before that you have to basically go back to the 16th century. Before that things were...looser. This laxity was problematic, leading to the canonization of some people that may not have existed. A number of people who had been declared saints that had dubious historicity were removed from the calendar of saints by the Catholic Church a number of years ago, but did not have their saintly status changed.