display post by one and only category

First post for me. Thanks for the assistance. What I’d like to do is generate pages for each category, but then have posts appear on only one of the category pages to avoid duplicate content. Short of going in and giving every post one and only one category, is there another way of selecting unique posts by only the first categoryID that the loop encounters?

So, if a post had categoryIDs of 3,19,34
I’d want to select the post for only categoryID=3.

thanks