It is very, very rare. I can't think of any cases in the US where it was proven in court. I can't think offhand of any cases like that anywhere. I can't remember the details of the recent case in South America.
I do believe one or two have been suspects, but they never could come up with sufficient evidence to press charges.
Just 2 recent cases come to mind, happened in the US.
2002 case -- Franciscan admits to killing priest, setting fire
2000 case -- Monk confesses to murder of nun in Kendall area