The Vth (in 964), IXth (a couple of times, in 1045 and in 1048), and now the XVIth (2013) have all done this. Unlike the previous two, the latest one has been due to old age and thus not so controversial.
What are we talking about?
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Resignations by Popes whose papal names were Benedict.
A Wikipedia list of Popes who resigned.
This day last year: Q.519