How are you achieving the double exposure on your shots?
I use a bunch of different iPhone apps for my photos. I believe you are referencing some of my recent photos that were created using the Hipstamatic app and choosing a lens/film combo that generates these. In this case, I believe the lens is a the Dali lens with one of the Ina films. Hipstamatic is a great app that shows the potential of the iPhone hardware. Really inspired interface and very clever use of in-app purchases.
Recommended further reading: Photojojo’s Ultimate Hipstamatic Guide and Big Wall of 336 Photos. Anybody disses the iPhone or Apple’s “lock-in” or whatever and I just point them to this app. That’s not a dis of another platform, OS or phone. I just use it as an example.