This is one way to combat spam comments if you can avoid duplicated comments are submitted on your posts, you can do some code modification as below in comments.php file. Why people post double comments because sometimes they are not aware that their comment is under moderation mode. Anyway, if you found your theme don’t have this option, then you can open comments.php and do some code modification.

1. Open comments.php and find the following code.

<p>
<input name="submit" type="submit" tabindex="5" value="<?php _e("Say it!"); ?>" />
</p>

2. Replace with

<p>
<blockquote>
Comment moderation is in use. Please do not submit your comment twice — it will appear shortly.
</blockquote>
<input name="submit" type="submit" tabindex="5" value="<?php _e("Say it!"); ?>" />
</p>