Fix duplicate entries in nav.html

This commit resolves #207.

_includes/nav.html

@@ -19,7 +19,7 @@
                     {%- endif -%}
                     <a href="{{ child.url | absolute_url }}" class="navigation-list-link{% if page.url == child.url %} active{% endif %}">{{ child.title }}</a>
                     {%- if child.has_children -%}
-                        {%- assign grand_children_list = site.html_pages | where: "parent", child.title | sort:"nav_order" -%}
+                        {%- assign grand_children_list = site.html_pages | where: "parent", child.title | where: "grand_parent", node.title | sort:"nav_order" -%}
                         <ul class="navigation-list-child-list">
                         {%- for grand_child in grand_children_list -%}
                           <li class="navigation-list-item {% if page.url == grand_child.url %} active{% endif %}">