The default alias for any user is blank. Because of that a blank username pulls up the first user entered that doesn't yet have an alias. If you give the first user added an alias the second one is assumed when no username is supplied, etc.
Fix it with this patch:
--- main.bak.inc
+++ main.inc
@@ -451,7 +451,7 @@
// query if user already registered
$sql_result = $DB->query("SELECT user_id, username, language, preferences
FROM ".get_table_name('users')."
- WHERE mail_host=? AND (username=? OR alias=?)",
+ WHERE mail_host=? AND (username=? OR (alias=? AND alias!=''))",
$host,
$user,
$user);