User activity warning in error Log

Hi I'm getting the following in my site log.

[Thu May 30 15:06:31 2013] [warn] [client 123.123.123.123] mod_fcgid: stderr: Site database error Multiple primary key defined for query ALTER TABLE myprefix_user_activity ADD PRIMARY KEY (active_ID) made by do_action('admin_init'), call_user_func_array, User_Activity->init, User_Activity->install, dbDelta, m_wpdb->query, referer: http://sitename.example.com/wp-admin/post.php?post=76&action=edit&message=1