Captcha problem with contact Widget

We us SI Captcha on the form we use for people creating there own blog on our service. We have now tried to install the contact plugin/ widget and get this error message Any additional code we can install to fix this error
Fatal error: Cannot redeclare _recaptcha_qsencode() (previously declared in /***********/*************/*****************/***********/wp-content/plugins/tantan-spam/plugin.php:519) in /home/************/************/**************/wp-content/plugins/contact-form/scripts/recaptchalib.php on line 55