It is unusual enough for any Arab ruler (or President for that matter) to abdicate or retire, but with the abdication of Sheikh Hamad of Qatar in his son's favor, Qatar will actually have two living former rulers at the same time. While I don't think it's historically unique (after the 1952 Egyptian revolution Egypt had two ex-kings, Farouq and his son Fuad, until Farouq died), it's probably more common in places where abdication is routine (as the Netherlands recently reminded us). (And Malaysia dosn't count, since the nine traditional rulers rotate the kingship among themselves every five years. Everybody's an ex-king.)
But Sheikh Khalifa bin Hamad Al Thani, father of Sheikh Hamad and grandfather of the new Amir, Sheikh Tamim, is still alive at the age of 80, having been overthrown by his son in 1995. After nearly a decade of living in France, he returned to Qatar in 2004. Sheikh Hamad, who is just 61, may also be around for some time to come.