I found out why the two textareas were showing up:
The comment_field is not supposed to be in the 'fields' array because it is a distinct parameter for the comment_form() function. It should be:
'url' => '<div class="grid-1-3">' . '<input id="url" placeholder="Website (optional)" name="url" type="text" value="' . esc_attr( $commenter['comment_author_url'] ) . '" size="30" /></div>',
)),
'comment_field' => '<div class="comment-con">' . '<textarea id="comment" placeholder="Your comment here. Be cool." name="comment" cols="45" rows="8" aria-required="true"></textarea>' . '</div>',
'comment_notes_after' => '',
); // line 73