Improve support when code occurrs multiple times

Signed-off-by: Danila Fedorin <danila.fedorin@gmail.com>

agda.rb

@@ -222,10 +222,18 @@ class Handler
                 # module HTML files. So, note the ID that we want to redirect,
                 # and pick a new unique ID to replace it with.
                 id = match[:id].to_s
-                uniq_id = "agda-unique-ident-#{id_counter}"
+                local_href = "#{html_file}##{id}"
+
+                # The same piece of Agda module code could have been included
+                # twice (e.g., a snippet first, then the surrounding code)
+                # In that case, don't assign it a new ID; prefer the earlier
+                # occurrence.
+                uniq_id = seen_hrefs.fetch(local_href) do
+                  new_id = "agda-unique-ident-#{id_counter}"
                   id_counter += 1
+                  seen_hrefs[local_href] = new_id
+                end
                 new_link['id'] = uniq_id
-                seen_hrefs["#{html_file}##{id}"] = uniq_id
               end
               new_link.content = content[relative_from...relative_to]