How does wordpress keep track of post id when post id is not used in permalinks?
Extending @Howdy_McGee answer. First you can not have two post with the same slug (Not title) which explained already in other answers. Second you can have page and post with same slug. But you will be able to access only page when your permalink structure is /%postname%/. And there is debate going on this ticket#13459 … Read more