<rss version="2.0" 
	xmlns:content="http://purl.org/rss/1.0/modules/content/"
	xmlns:dc="http://purl.org/dc/elements/1.1/"
	xmlns:atom="http://www.w3.org/2005/Atom"
	>
    <channel>

        <title>{{ feed_config.title }}</title>
        <description>{{ feed_config.description }}</description>

        {% if local_config.get('base_url') %}
        <link>{{ local_config.get('base_url') }}</link>
        {% endif %}

        <atom:link href="{{ local_config.get('base_url') }}/feed" rel="self" type="application/rss+xml" />

        {% if feed_config.updated %}
        <lastBuildDate>{{ feed_config.updated|date("D, d M Y H:i:s O") }}</lastBuildDate>
        {% endif %}

        {% for post in posts %}
        <item>
          <title>{{ post.getSubject() }}</title>
          <link>{{ post.getUrl() }}</link>
          <pubDate>{{ post.getCreatedAt()|date("D, d M Y H:i:s O") }}</pubDate>
          <dc:creator>{{ post.get('name') }}</dc:creator>
          <guid>{{ post.getUrl() }}</guid>
          <description>{{ post.getBodyAsHtml() }}</description>
        </item>
        {% endfor %}
    </channel>
</rss>
<rss version="2.0" 
	xmlns:content="http://purl.org/rss/1.0/modules/content/"
	xmlns:dc="http://purl.org/dc/elements/1.1/"
	xmlns:atom="http://www.w3.org/2005/Atom"
	>
    <channel>

        <title>{{ feed_config.title }}</title>
        <description>{{ feed_config.description }}</description>

        {% if local_config.get('base_url') %}
        <link>{{ local_config.get('base_url') }}</link>
        {% endif %}

        <atom:link href="{{ local_config.get('base_url') }}/feed" rel="self" type="application/rss+xml" />

        {% if feed_config.updated %}
        <lastBuildDate>{{ feed_config.updated|date("D, d M Y H:i:s O") }}</lastBuildDate>
        {% endif %}

        {% for post in posts %}
        <item>
          <title>{{ post.getSubject() }}</title>
          <link>{{ post.getUrl() }}</link>
          <pubDate>{{ post.getCreatedAt()|date("D, d M Y H:i:s O") }}</pubDate>
          <dc:creator>{{ post.get('name') }}</dc:creator>
          <guid>{{ post.getSluglessUrl() }}</guid>
          <description>{{ post.getBodyAsHtml() }}</description>
        </item>
        {% endfor %}
    </channel>
</rss>

Use the Slugless URL as the GUID
Use the Slugless URL as the GUID

This prevents feed readers from thinking posts are new if their title has been modified.

Downside: Everything will show up as new again the first time.