diff --git a/example/source/shared-subheadings/page-a.html.md b/example/source/shared-subheadings/page-a.html.md
new file mode 100644
index 00000000..037eb3be
--- /dev/null
+++ b/example/source/shared-subheadings/page-a.html.md
@@ -0,0 +1,14 @@
+---
+layout: layout
+---
+
+# Shared subheadings - Page A
+
+A page that shares headings with the other pages in this folder,
+to check that the heading IDs are computed properly
+
+## A subheading
+
+### A h3 subheading
+
+## Another subheading
diff --git a/example/source/shared-subheadings/page-b.html.md b/example/source/shared-subheadings/page-b.html.md
new file mode 100644
index 00000000..1acba227
--- /dev/null
+++ b/example/source/shared-subheadings/page-b.html.md
@@ -0,0 +1,14 @@
+---
+layout: layout
+---
+
+# Shared subheadings - Page B
+
+A page that shares headings with the other pages in this folder,
+to check that the heading IDs are computed properly
+
+## A subheading
+
+### A h3 subheading
+
+## Another subheading
diff --git a/lib/govuk_tech_docs.rb b/lib/govuk_tech_docs.rb
index 4cacd978..37843991 100644
--- a/lib/govuk_tech_docs.rb
+++ b/lib/govuk_tech_docs.rb
@@ -18,7 +18,6 @@
 require "govuk_tech_docs/page_review"
 require "govuk_tech_docs/pages"
 require "govuk_tech_docs/tech_docs_html_renderer"
-require "govuk_tech_docs/unique_identifier_extension"
 require "govuk_tech_docs/unique_identifier_generator"
 require "govuk_tech_docs/warning_text_extension"
 require "govuk_tech_docs/api_reference/api_reference_extension"
@@ -62,7 +61,6 @@ def self.configure(context, options = {})
 
     config_file = ENV.fetch("CONFIG_FILE", "config/tech-docs.yml")
     context.config[:tech_docs] = YAML.load_file(config_file).with_indifferent_access
-    context.activate :unique_identifier
     context.activate :warning_text
     context.activate :api_reference
 
diff --git a/lib/govuk_tech_docs/tech_docs_html_renderer.rb b/lib/govuk_tech_docs/tech_docs_html_renderer.rb
index f9f2d614..3eef19e3 100644
--- a/lib/govuk_tech_docs/tech_docs_html_renderer.rb
+++ b/lib/govuk_tech_docs/tech_docs_html_renderer.rb
@@ -1,4 +1,5 @@
 require "middleman-core/renderers/redcarpet"
+require "govuk_tech_docs/unique_identifier_generator"
 
 module GovukTechDocs
   class TechDocsHTMLRenderer < Middleman::Renderers::MiddlemanRedcarpetHTML
@@ -10,12 +11,18 @@ def initialize(options = {})
       super
     end
 
+    def preprocess(document)
+      @unique_identifier_generator = UniqueIdentifierGenerator.new
+
+      document
+    end
+
     def paragraph(text)
       @app.api("<p>#{text.strip}</p>\n")
     end
 
     def header(text, level)
-      anchor = UniqueIdentifierGenerator.instance.create(text, level)
+      anchor = @unique_identifier_generator.create(text, level)
       %(<h#{level} id="#{anchor}">#{text}</h#{level}>\n)
     end
 
diff --git a/lib/govuk_tech_docs/unique_identifier_extension.rb b/lib/govuk_tech_docs/unique_identifier_extension.rb
deleted file mode 100644
index bb50e943..00000000
--- a/lib/govuk_tech_docs/unique_identifier_extension.rb
+++ /dev/null
@@ -1,13 +0,0 @@
-module GovukTechDocs
-  class UniqueIdentifierExtension < Middleman::Extension
-    def initialize(app, options_hash = {}, &block)
-      super
-
-      app.before do
-        UniqueIdentifierGenerator.instance.reset
-      end
-    end
-  end
-end
-
-::Middleman::Extensions.register(:unique_identifier, GovukTechDocs::UniqueIdentifierExtension)
diff --git a/lib/govuk_tech_docs/unique_identifier_generator.rb b/lib/govuk_tech_docs/unique_identifier_generator.rb
index 62b932c5..6006fcb7 100644
--- a/lib/govuk_tech_docs/unique_identifier_generator.rb
+++ b/lib/govuk_tech_docs/unique_identifier_generator.rb
@@ -1,15 +1,11 @@
-require "singleton"
-
 module GovukTechDocs
   class UniqueIdentifierGenerator
-    include Singleton
-
     Anchor = Struct.new(:id, :level)
 
     attr_reader :anchors
 
     def initialize
-      reset
+      @anchors = []
     end
 
     def create(id, level)
@@ -28,10 +24,6 @@ def create(id, level)
       anchor
     end
 
-    def reset
-      @anchors = []
-    end
-
   private
 
     def prefixed_by_parent(anchor, level)
diff --git a/spec/govuk_tech_docs/tech_docs_html_renderer_spec.rb b/spec/govuk_tech_docs/tech_docs_html_renderer_spec.rb
index 83723aad..fcba3f8f 100644
--- a/spec/govuk_tech_docs/tech_docs_html_renderer_spec.rb
+++ b/spec/govuk_tech_docs/tech_docs_html_renderer_spec.rb
@@ -95,4 +95,51 @@ def hello_world
       end
     end
   end
+
+  describe "#render unique heading IDs" do
+    it "Automatically assigns an ID to the heading" do
+      output = processor.render <<~MARKDOWN
+        # A heading
+        ## A subheading
+      MARKDOWN
+
+      expect(output).to include('<h1 id="a-heading">A heading</h1>')
+      expect(output).to include('<h2 id="a-subheading">A subheading</h2>')
+    end
+
+    it "Ensures IDs are unique among headings in the page" do
+      output = processor.render <<~MARKDOWN
+        # A heading
+        ## A subheading
+        ### A shared heading
+        ### A shared heading
+        ### A shared heading
+        ## Another subheading
+        ### A shared heading
+      MARKDOWN
+
+      # Fist heading will get its ID as a normal heading
+      expect(output).to include('<h3 id="a-shared-heading">A shared heading</h3>')
+      # Second occurence will be prefixed by the previous heading to ensure uniqueness
+      expect(output).to include('<h3 id="a-subheading-a-shared-heading">A shared heading</h3>')
+      # Third occurence will be get the prefix, as well as a numeric suffix, to keep ensuring uniqueness
+      expect(output).to include('<h3 id="a-subheading-a-shared-heading-2">A shared heading</h3>')
+      # Finally the last occurence will get a prefix, but no number as it's in a different section
+      expect(output).to include('<h3 id="another-subheading-a-shared-heading">A shared heading</h3>')
+    end
+
+    it "Does not consider unique IDs across multiple renders" do
+      processor.render <<~MARKDOWN
+        # A heading
+        ## A subheading
+      MARKDOWN
+
+      second_output = processor.render <<~MARKDOWN
+        # A heading
+        ## A subheading
+      MARKDOWN
+
+      expect(second_output).to include('<h2 id="a-subheading">A subheading</h2>')
+    end
+  end
 end
diff --git a/spec/govuk_tech_docs/unique_identifier_generator_spec.rb b/spec/govuk_tech_docs/unique_identifier_generator_spec.rb
index 074ff1a0..6c79f851 100644
--- a/spec/govuk_tech_docs/unique_identifier_generator_spec.rb
+++ b/spec/govuk_tech_docs/unique_identifier_generator_spec.rb
@@ -1,6 +1,5 @@
 RSpec.describe GovukTechDocs::UniqueIdentifierGenerator do
-  subject { described_class.instance }
-  before { subject.reset }
+  subject { described_class.new }
 
   describe "#create" do
     it "lower-cases the text" do
@@ -66,13 +65,4 @@
       end
     end
   end
-
-  describe "#reset" do
-    it "clears the list of existing anchors" do
-      subject.create("An Anchor", 1)
-      expect(subject.anchors).to_not be_empty
-      subject.reset
-      expect(subject.anchors).to be_empty
-    end
-  end
 end