I don't think the rule is needed. Nominations and category selections should keep most books out of repeated nominations. I thought it was the nomination and voting process itself that was supposed to monitor appropriateness of titles for a given category within the bounds of what the category nominator defines.
If I went to the library and went to the romance section at least four of the nominated books wouldn't have been on the shelf there. Genres are open to interpretation and I like the fact that we nominate oddities and get a lot of opinions. Is it always going to work? No, but the process that is there for nominating and voting should get us to at least a partial-majority wanting to read a book, independent of what we choose to label it as.