This is would probably have to be a plugin. This could only really be reliably done on a page that requires a user to be logged in and you can't just register new accounts. Also someone could just copy the text the first time and then your security is moot. Nevertheless, the plugin would have to track the page ID and the user ID and when the visit the page a row gets added to table. Then you could have a page template where you check that table and if the row exists for that page id and user you do whatever to the content otherwise display it as normal.