Reset UniqueIdentifierGenerator between each render

Prevents IDs to be misinterpreted as duplicated across the rendering of different pages sharing headings

lib/govuk_tech_docs/tech_docs_html_renderer.rb

@@ -11,6 +11,12 @@ def initialize(options = {})
       super
     end
 
+    def preprocess(document)
+      UniqueIdentifierGenerator.instance.reset
+
+      document
+    end
+
     def paragraph(text)
       @app.api("<p>#{text.strip}</p>\n")
     end

spec/govuk_tech_docs/tech_docs_html_renderer_spec.rb

@@ -109,7 +109,7 @@ def hello_world
       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
@@ -130,5 +130,19 @@ def hello_world
       # 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