$wpdb->insert inserting two rows in database

Try something like this

function insert_data_into_table () {
        global $wpdb;
        $table_name = $wpdb->prefix . "your_table_name";

        //Check if table exists
        if($wpdb->get_var("show tables like '$table_name'") != $table_name) :

            //if not, create the table   
            $sql = "CREATE TABLE " . $table_name . " (
            (...)
            ) ENGINE=InnoDB;";

            require_once(ABSPATH . 'wp-admin/includes/upgrade.php');
            dbDelta($sql);
        else:
          foreach($emails as $mail):
              $insert = "INSERT INTO  " . $table_name . "
                (post_id, post_id, email, post_id) 
                VALUES ($post_id, $user_id, email['content'], 1);"
              $results = $wpdb->query( $insert );
          endforeach;
        endif;
    }

I’m assuming that $email is an array containing some data? If you want to save the entire array, then you should serialize() the array.