I've seen this a few times on my board and have not been able to find the cause or code area. What is happening is customers can create multiple accounts using the same email address, address, etc. and this totally screws up their shopping carts (of course).
The only reference I found for this is in OSCommerce 3.0 (beta) and it was a code error that I can't find in OSCMax or OSCommerce.
I am running the most recent version of OSCMax 2.0.25 and everything else appears to be running well - although my /admin PHP errors file is showing many 'Notices' related to:
PHP Notice: Undefined variable: listing in /usr.../admin/customers.php on line 1022
PHP Notice: Constant AFFILIATE_NOTIFY_AFTER_BILLING already defined in /usr..../admin/includes/application_top.php on line 69
PHP Notice: Constant TYPE_BELOW already defined in /usr...../admin/includes/languages/english/customers.php on line 40
And a few more p these are only notices and not serious.
However I haven't yet figured out how to turn on the PHP logging of customer side - working on that after lunch - but was hoping there was a simple fix for my customers.