If for some reason an IPN message about successful transaction from PayPal to OBAS gets resent by PayPal, OBAS responces to that second message with Internal Error 500 despite the first IPN message was correctly answered with OK 200.
As a result, PayPal retries the message again and again.
500 error logged in
188.8.131.52 - - [27/Oct/2014:09:54:59 +0000] "POST /hspc/common/01/ref_no,2492.77806.188581-2014-10-27-23684cb68982441/template,paypalap/action,notify HTTP/1.1" 500 - "-" "-"
Starting with OBAS 4.5.6, OBAS replies with 200 on every IPN message about a successful transaction, no matter when the transaction actually took place.
To resolve the issue, please upgrade to 4.5.6 as described in How to upgrade OBAS system to the latest available version.