A logged in user can't register another participant if this participant has the same email as an existing user

That's an edge case:

I'm logged in as xavier, there is another drupal user john@doe.com (my colleague).

I click on the registration link, "Welcome Xavier dutoit. (Not Xavier dutoit, or want to register a different person?)"

I click I want register another person (/civicrm/event/register?&cid=0&reset=1&id=42),

start filling the info for my colleague John, got an error message: "he email john@doe.com already exists in the database."

The test is in CRM/Event/Form/Registration/Register.php
require_once 'CRM/Core/BAO/UFMatch.php';
if ( CRM_Core_BAO_UFMatch::isDuplicateUser( $email ) ) {
$errors["email-{$self->_bltID}"] = ts( 'The email %1 already exists in the database.',
array( 1 => $email ) );

I fail to see the use case of this test (we already test the email duplicate or not, and the contact dupe...

Anyway, I've added a patch that doesn't do that test only if the user is not registering someone else