In the south of [name_f]Scotland[/name_f] [name_m]Alistair[/name_m] and [name_m]Alister[/name_m] are both equally common, but in the north, [name_m]Alasdair[/name_m] is definetly most popular, though all three are perfectly normal everywhere. I’ve seen a couple of other variations, but they aren’t nearly as common, and I’ve never seen it spelled with two Ls as a first name.