We don’t need to manage this tags list in two places.

.eleventy.js

@@ -41,10 +41,11 @@ module.exports = function(eleventyConfig) {
     return Math.min.apply(null, numbers);
   });
 
-  eleventyConfig.addFilter("filterTagList", tags => {
-    // should match the list in tags.njk
+  function filterTagList(tags) {
     return (tags || []).filter(tag => ["all", "nav", "post", "posts"].indexOf(tag) === -1);
-  })
+  }
+
+  eleventyConfig.addFilter("filterTagList", filterTagList)
 
   // Create an array of all tags
   eleventyConfig.addCollection("tagList", function(collection) {
@@ -53,7 +54,7 @@ module.exports = function(eleventyConfig) {
       (item.data.tags || []).forEach(tag => tagSet.add(tag));
     });
 
-    return [...tagSet];
+    return filterTagList([...tagSet]);
   });
 
   // Copy the `img` and `css` folders to the output

posts/thirdpost.md

@@ -4,6 +4,7 @@ description: This is a post on My Blog about win-win survival strategies.
 date: 2018-08-24
 tags:
   - second tag
+  - posts with two tags
 layout: layouts/post.njk
 ---
 Leverage agile frameworks to provide a robust synopsis for high level overviews. Iterative approaches to corporate strategy foster collaborative thinking to further the overall value proposition. Organically grow the holistic world view of disruptive innovation via workplace diversity and empowerment.

tags-list.njk

@@ -4,7 +4,7 @@ layout: layouts/home.njk
 ---
 <h1>Tags</h1>
 
-{% for tag in collections.tagList | filterTagList %}
+{% for tag in collections.tagList %}
   {% set tagUrl %}/tags/{{ tag | slug }}/{% endset %}
   <a href="{{ tagUrl | url }}" class="post-tag">{{ tag }}</a>
 {% endfor %}