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wp_list_comments() will append an end tag to your comment

On every call to your call back from wp_list_comments(), there will be an end tag appended, this tag can be either </li> or </div> depending on your style parameter. Therefore, in your call back function, don’t put your own end tag there. For example, my call back function was like this: function custom_comments($comment, $args, $depth) [...]