There is a short story circa 1950's with the following plot: the main hero (young men) gets (maybe via a post) a kind of do-it-yourself kit that allows one to create various life-forms including human. He starts with simple life-forms then tries to create more difficult ones. Finally he decides to use the kit to create a copy of a girl who he loves (she doesn't love him). To prepare better, he decides to create first his own copy. When he creates a copy, an officer appear and explains that the kit was delivered by mistake and it belongs to a different time. So he decides to get back the kit and kill a copy. However, the copy claims that it is the original and the officer believe the copy, thus killing the original hero.
I read it about 15 years ago in Russian translation (I believe it's a short story by American author), most probably in a series of other short stories. However, now I wasn't able to find this story using search engines neither in Russian nor in English.