WordPress appends a number to the slug when the database already contains a duplicate slug. It will append the number when the duplicate post or page has been moved to the trash. When Pages or Posts are “deleted” they are simply moved to a Trash folder. Items within your Trash folders can be restored or permanently deleted within 30 days of their original trashing. This is specially useful if you need to retrieve a Page or Post within those 30 days, or if you accidentally deleted an item. If you create a Page or a Post with the same name as a deleted one, WordPress will append a number to it since one already exists in the database. This may not look that nice in the permalinks.
Simply access the Trash Folder for Pages or for Posts and “Permanently Delete” the item before creating a new one. For more info on WordPress Trash, please see the following: http://en.support.wordpress.com/trash/