Comments Plus Install Error

Hi are you able to help with an error I'm getting with Comments Plus plugin. Have tried deactivating all other plugins & switching themes, but still getting this error message:

I have also changed the theme to twenty eleven, same error.

Fatal error: Cannot redeclare class OAuthException in /public_html/wp-content/plugins/comments-plus/lib/external/twitter/OAuth.php on line 8

thanks for your help