"Whether the authors 'invented' these ideas, or were the first to use them, isn't significant. It's how they used them, clothed them in story and made thme live, which matters. Often a number of writers are drawn to explore the same ideas at the same time, because these are questions which are very much in the public consciousness... Writers may consciously borrow ideas from books they've read wanting to argue or put their own spin on them, or they may absorb them unconssciously from the common culture. Ideas get recycled, reused, and some become such common currency within SF that their origins are forgotten."--Lisa Tuttle, Writing Fantasy and Science Fiction
Note: A solid book on the field. The unique strength or value of this book is its analyses. Her advice is familiar but when she breaks down her work or those of others, you'll want to listen.