Membership Plugin won't process Authorize.net payment.

I am running Membership 3.5.beta.7 and I cannot complete payment. I get the following under POST in Firebug Console: <b>Fatal error</b>: Cannot redeclare class AuthorizeNetException in <b>/home/content/55/11019955/html/wp-content/plugins/membership/classes/Authorize.net/AuthorizeNet.php</b> on line <b>29</b>

Seems like a syntax error to me? Help?