Detect duplicate keys in frontmatter YAML

spec/frontmatter_spec.rb

@@ -0,0 +1,15 @@
+require 'spec_helper'
+
+RSpec.describe '.md files' do
+  Dir["#{REPO_ROOT}/content/**/*.md"].sort.each do |path|
+    page = path.split(REPO_ROOT.to_s).last
+
+    describe page do
+      it 'does not include duplicate keys in YAML frontmatter' do
+        _, frontmatter, _content = File.read(path).split('---', 3)
+
+        expect(YAMLHelper.duplicate_keys(frontmatter)).to be_empty
+      end
+    end
+  end
+end

spec/spec_helper.rb

@@ -9,6 +9,7 @@
   config.disable_monkey_patching!
 end
 
+REPO_ROOT = Pathname.new(File.expand_path('../..', __FILE__))
 SITE_ROOT = Pathname.new(File.expand_path('../../_site', __FILE__))
 
 def file_at(path)

spec/support/yaml_helper.rb

@@ -0,0 +1,35 @@
+module YAMLHelper
+  module_function
+
+  # from https://stackoverflow.com/a/55705853
+  def duplicate_keys(file_or_content)
+    yaml = file_or_content.is_a?(File) ? file_or_content.read : file_or_content
+    duplicate_keys = []
+
+    validator = ->(node, parent_path) do
+      if node.is_a?(Psych::Nodes::Mapping)
+        children = node.children.each_slice(2) # In a Mapping, every other child is the key node, the other is the value node.
+        duplicates = children.map { |key_node, _value_node| key_node }.group_by(&:value).select { |_value, nodes| nodes.size > 1 }
+
+        duplicates.each do |key, nodes|
+          duplicate_key = {
+            file: (file_or_content.path if file_or_content.is_a?(File)),
+            key: parent_path + [key],
+            occurrences: nodes.map { |occurrence| "line: #{occurrence.start_line + 1}" },
+          }.compact
+
+          duplicate_keys << duplicate_key
+        end
+
+        children.each { |key_node, value_node| validator.call(value_node, parent_path + [key_node&.value].compact) }
+      else
+        node.children.to_a.each { |child| validator.call(child, parent_path) }
+      end
+    end
+
+    ast = Psych.parse_stream(yaml)
+    validator.call(ast, [])
+
+    duplicate_keys
+  end
+end