WordPress will not allow you, using its default comment process, to insert empty comments into the database. This is enforced by wp_handle_comment_submission() with the following unconditional code:
if ( '' == $comment_content ) {
return new WP_Error( 'require_valid_comment', __( '<strong>ERROR</strong>: please type a comment.' ), 200 );
}
If you insist on using comments, you will need to supply some non-empty value for $comment_content, which is most simply accomplished by inserting some whitespace content into the comment field itself.
As for duplicates, since WordPress 4.4.0 you can allow duplicate comments by using the duplicate_comment_id filter hook. As per the docs:
Filters the ID, if any, of the duplicate comment found when creating a new comment.
Return an empty value from this filter to allow what WP considers a duplicate comment.
For example, use the following code to unconditionally allow duplicate comments:
add_filter('duplicate_comment_id', '__return_false');
Hope this helps. 🙂