If I use the format domain.com/category/post-title What happens if I create two posts with the same title?
I'm pretty sure WordPress will throw an exception and tell you that you already have a post named that. But, I'm not 100% sure.
NO problem if it have same title... wordpress will change the slug automatically... if u use domain.com/post-name type perma then wordpress will name the 2nd post with a "-2" as the the end of the post's slug...