Registration button not displayed on event info if already registered

The civicrm/event/info link does display the "It looks like you are already registered..." status message for authenticated users when it detects that their contact is already registered in a "counted" status. However, that status message was missing the additional text / link:

"You can also register another participant."

I've added that to CRM/Event/Page/EventInfo.php.

I was not able to replicate the case where a logged in user who was already registered did NOT see the status message AND did not have the Register button. If you can provide steps to replicate that - we should try and fix it by making sure the status message always displays on event/info when we know user is registered (please debug and attach a patch for any fix needed).

— original post —
The civicrm/event/info link doesn't contain the "register" button/link if the user is already registered. However if you try going to the /civicrm/event/register page, you are redirected to the info page with a super useful message:

Oops. It looks like you are already registered for this event. If you want to change your registration, or you feel that you've gotten this message in error, please contact the site administrator. You can also register another participant (link with cid=0).

Instead of not displaying the button silently, it should display that message.

Rationale:

• it allows to provide the link for an admin
• it avoids questions "is it disabled for everyone or only for me?"