You can achieve this using the placeholders and accomplish what you want. My Event format has the following:

#_ADDBOOKINGFORM_IF_NOT_REGISTERED
#_REMOVEBOOKINGFORM_IF_REGISTERED

If they’re already registered, it gives them the form to cancel their registration. If they aren’t already registered, then they get the form for signing up.

If you only use the first one, it will essentially remove the signup form if they’ve already signed up. That should get you want you need. Just find the space in your form where there’s the placeholder #_ADDBOOKINGFORM and add the _IF_NOT_REGISTERED to it.

What you want is to check if the user already registered *after* he tries to register again, but before the info is entered in the db.
The eme_eval_booking_form_filter is what you want, but I just realized it needs to be changed: for now the person doing the booking is first recorded in the db and only afterwards the evaluation is done. When the eval fails, a person is in the DB for no reason. To keep the old behavior, I added a new filter called eme_eval_booking_filter.
Example usage (put this in your theme functions.php when the next release is there):

add_action('eme_eval_booking_filter','do_my_stuff');
function do_my_stuff($event) {
    $event_id=$event['event_id'];
    $already_registered=0;

    // only do a specific event
    if ($event_id != 5) return;

    // get all attendees
    $attendees = eme_get_attendees_for($event_id);
    foreach ( $attendees as $attendee ) {
       // $attendee['person_email'], $attendee['person_name']
       $bookerEmail = eme_sanitize_html(stripslashes_deep($_POST['bookerEmail']));
       $bookerName = eme_sanitize_html(stripslashes_deep($_POST['bookerName']));
       if ($bookerEmail == $attendee['person_email'] && $bookerName == $attendee['person_name'])
          $already_registered=1;
    }

    if (!$already_registered) {
       // eval is success
       return array(0=>1,1=>'');
    } else {
       // bad form entry
       return array(0=>0,1=>'Already registered');
    }
}

And also: check the “quotes” for the add_action line in your functions.php. I saw that they were not the correct quotes, so … corrected the example again.
Edit: tested this example code and it works as intended. Also added it to the doc as an example usage for the filters.

If you want it for all events, just leave the check for the event id out of it. So remove the line
if ($event_id != 5) return;

Did you test this? The example code posted still takes the event id into account, so it should work just fine.

Just tested this and it works just fine: it prevents double registrations for all events. If it doesn’t work for you, post the code you use.

Make sure that the lines are in fact (check the “>” character in your code)

return array(0=>1,1=>'');

In your posted code it was (I corrected the code) the html equiv of the “>” character was being used, so maybe that is also the case in your code?

(probably because of a copy/paste with your browser). Also with the other return-line.

Edit: in fact, check the whole function to see if it is syntactically correct.

I don’t see anything wrong here. Can you send an admin account so I can check on your side?

For completeness: was just wrongfully pasted in the functions.php file, the code works as expected.