cannot redeclare class TCPDF when submitting order

[tonyl09]

After upgrading from 1.6.1.6 to 1.6.1.10 customers do not see confirmation that their order is placed.

 

When errors are displayed, a customer will see the notice "Cannot redeclare class TCPDF in .../tools/tcpdf/tcpdf.php on line 161."

 

Their order is successful, and authorize.net charges them successfully.

 

The Customer and Administrator emails are generated, and the customer receives attached PDFs with their email. The module that creates the 

 

How can I troubleshoot this?

 

Thank you