Check for duplicate id attributes in HTML

It’s invalid HTML to have multiple elements with the same `id`
attribute. Plus it can cause trouble for Javascripts, and make anchor
linking unpredictable.

tests/conftest.py

@@ -3007,6 +3007,8 @@ def get_url(
             if _test_for_non_smart_quotes:
                 ClientRequest.test_for_non_smart_quotes(page)
 
+            ClientRequest._test_for_duplicate_ids(page)
+
             return page
 
         @staticmethod
@@ -3149,6 +3151,14 @@ def test_for_non_smart_quotes(page):
                     "'" in el.text or '"' in el.text
                 ), f"Non-smart quote or apostrophe found in <{el.name}>: {normalize_spaces(el.text)}"
 
+        @staticmethod
+        def _test_for_duplicate_ids(page):
+            ids = [element["id"] for element in page.select("*[id]")]
+            for id in ids:
+                assert ids.count(id) == 1, f"Duplicate id `{id}` found on these elements:\n    " + ", ".join(
+                    f"<{element.name}>" for element in page.select(f"*[id='{id}']")
+                )
+
     return ClientRequest