I am having the same issue. It is happening with everyone who goes through PayPal whether existing customer or not. Here's an example:
PHP v5.3.8 :: WordPress® v3.2.1 :: s2Member® v111029 :: s2Member® Pro v111029
Memory 41.54 MB :: Real Memory 42.00 MB :: Peak Memory 41.67 MB :: Real Peak Memory 42.00 MB
medicalfitness.org/?s2member_paypal_return=1&
User-Agent: Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 6.1; WOW64; Trident/4.0; SLCC2; .NET CLR 2.0.50727; .NET CLR 3.5.30729; .NET CLR 3.0.30729; Media Center PC 6.0; .NET4.0C)
array (
'transaction_subject' => 'New Professional Membership at $225',
'payment_date' => '04:16:04 Nov 18, 2011 PST',
'txn_type' => 'subscr_payment',
'subscr_id' => 'I-PUGMCW38BY2H',
'last_name' => '*******',
'option_selection1' => 'medicalfitness.org',
'option_selection2' => '108.74.16.37',
'residence_country' => 'US',
'item_name' => 'New Professional Membership at $225',
'payment_gross' => '225.00',
'mc_currency' => 'USD',
'business' =>
'********@medicalfitness.org',
'payment_type' => 'instant',
'protection_eligibility' => 'Ineligible',
'payer_status' => 'verified',
'payer_email' => '***********',
'txn_id' => '6MD03007GC6412438',
'receiver_email' =>
'*********@medicalfitness.org',
'first_name' => 'Bruce',
'option_name1' => 'Originating Domain',
'invoice' => '4ec64b91919a2~108.74.16.37',
'payer_id' => '2GRFM48VBRKB4',
'option_name2' => 'Customer IP Address',
'receiver_id' => 'F6ZLV2H5WSSR8',
'item_number' => '1:professional',
'payment_status' => 'Completed',
'payment_fee' => '5.93',
'mc_fee' => '5.93',
'mc_gross' => '225.00',
'custom' => 'medicalfitness.org',
'charset' => 'windows-1252',
's2member_log' =>
array (
0 => 'Return-Data received on: Fri Nov 18, 2011 7:17:20 am EST',
1 => 's2Member POST vars verified through a POST back to PayPal®.',
2 => 's2Member originating domain ( `$_SERVER["HTTP_HOST"]` ) validated.',
3 => 'Page Expired. Duplicate Return-Data.',
4 => 's2Member `txn_type` identified as ( `web_accept|subscr_signup|subscr_payment` ).',
5 => 'Page Expired. Redirecting Customer to the Home Page.',
),
'subscr_gateway' => 'paypal',
's2member_paypal_return_tra' => 'fnIyOjlna1pZVk1wZUZJSHFzVmlHelVvbXJXY3FKRld6M2F2fNAduDjEM6aNFR39d6qFwz4Pk1Aqkx9WWUXSyupfEaNbiHJ6VRV91vPnNKWsHXymkd5K66KsiAyXVNPHcGHR7o9HaeGZJiKGxr5fimv6Enhq_SikUsunN6XWiJ2U72chm7AzBtQkjVFxqLpQTT6KPuYvYmlk_me9p8EYiCae9oEIJ8Ui0pVjOLIY8HFDpOdCEiPOE4gkxD9UWoXvPsC52hjnVsARXGQfkGCIwgXWyWZKlLZ4oe_SbRU7Lki9sFCsTtXAgoU9srL0Z9E7wvn0GKZeiT5WD08G1bCJiw6WD87nphJZ-Bh3VEBwJcwSZPBbWWyJ3SvAuc-agsHSaVM4HRIP1zkEJbTPBEOq2roD1tyLOgTDErVHh-i6heg-HzBvmA',
)
Button Code: [s2Member-PayPal-Button level="1" ccaps="professional" desc="New Professional Membership at $225" ps="paypal" lc="" cc="USD" dg="0" ns="1" custom="medicalfitness.org" ta="0" tp="0" tt="D" ra="225.00" rp="1" rt="Y" rr="0" rrt="" rra="1" image="default" output="button" /]
Thanks!