I’ve just found out that Kästner’s DAS DOPPELTE LOTTCHEN has been translated into English as LOTTIE AND LISA:
Unfortunately, the book seems to be out of print, but I’m sure you can get it used, if you’re interested in Kästner’s novel.
My favourite film adaptation is the old black and white German film with Jutta and Isa Günther. All the women in my family seem to love this version best. 🙂