Solution
Hello everyone
to stop multiple customer id / Guest id with same email PRESTASHOP 1.6
Go in this file \publicHTML\classes\Customer.php
remplace variable $ignore_guest true to false in function parameters like this
Lines 392-393
// public static function customerExists($email, $return_id = false, $ignore_guest = true)
public static function customerExists($email, $return_id = false, $ignore_guest = false)
Enjoy