Actually I've just walked into this trap myself. With novel #1 under my belt, I already had the theme for #2. Started writing #2, which shares some locations and one or two characters with #1, but is not a sequel. I'm about a quarter of the way into this one, and someone just gave me the idea for #3, which is a sequel to #1, and which is trying very hard to lure me away from the one I'm working on.
