fatal error to appointments plugin

I want to install appointments onto my site but when I activate it, it keeps crashing the whole site and I have to delete the plugin using my ftp software.

The error it brings up is

Fatal error: Cannot redeclare class google_signer in /home/digitalm/public_html/wp-content/plugins/appointments/includes/gcal/auth/Google_Signer.php on line 28

Not sure what to do. Took the plugin off my site completely