Yeah I got a call Tues as well
I've read on CC that a few other ppl got them so I'm thinking they did it so those who hadn't paid deposits to other places wouldn't have to, and to raise enthusiam for Duke if other students get off other waitlists.
Don't worry though, since this is before the deadline for accepted kids to reply, I'm pretty sure the majority of spots will be offered Mid-May. Good luck!!!
(now it is time to watch LOST)