Das doppelte Lottchen translates to the double Lottie which is a 1949 novel by Erich Kästner, about twin girls separated at birth who meet at summer camp. Sound familiar?!
Two nine-year-old girls—rude Lisa Palfy, originally Luise Palffy from Vienna, and respectful shy Lottie Horn originally Lotte Körner from Munich—meet in a summer camp in Bohrlaken on Lake Bohren, where it is discovered that they are identical twins whose parents divorced, each keeping one of the girls. While the girls are younger in the book than the movie, they tried to give Susan and Sharon different personalities like the book. It seems that they were a lot more subtle than Lisa and Lottie though with slight accents and nail biting.
The girls decide to trade places at the end of the summer so that Lottie will have a chance to get to know her father and Lisa will get to meet her mother. The adults are surprised at the changes in each of the girls after they return from camp in the book. In the movie, the parents seem to notice their small differences as well.
The story was originally supposed to be a movie. In 1942, when for a brief time Kästner was allowed by the Nazi authorities to work as a screenwriter, he proposed it to Josef von Báky, under the name “The Great Secret”, but before he could proceed the Nazis once again forbade him to work. Damn Nazis. After the war, Kästner worked the idea into a highly successful book. The story was then turned into other medias with Disney’s The Parent Trap being the most popular. Also, notice that the girls in both the movie and the book are blonde.