Error in mdpf.php line 65

Fatal error: Cannot redeclare class mPDF in /home/wifilights/wifilights.co.nz/wp-content/plugins/woocommerce-pdf-invoices/mpdf/mpdf.php on line 65

I am getting this error on completion of a sale on my site. The site still sees the order and sends everyone the correct emails etc, however the customer then doesn’t go to the next page with payment complete message.

Any idea why this might be happening?

https://wordpress.org/plugins/woocommerce-pdf-invoices/