I actually didn't get the e-mail even though I have a visit scheduled.
Duke really wants people to come visit. I'm guessing maybe they saw less people coming to campus to interview than they'd hoped. It's not like it would increase their yield to offer a break on app fees.
It seems like they do offer quite a few waivers. When I was at Weekend for Women last year, they offered [full] waivers to everyone who attended and it seems like there are a lot of waivers listed in their application.