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<?php
/** Plugin Name: (#90328) Login with E-Mail address */
function login_with_email_address( &$username ) {
    $user = get_user_by( 'email', $username );
    if ( !empty( $user->user_login ) )
        $username = $user->user_login;
    return $username;
}
add_action( 'wp_authenticate','login_with_email_address' );