Add test to assert unique ids in singlehtml output

Since the singlehtml aggregates all doc files into a single html page
during the write step, and the ids must be unique for proper link
anchoring, add test that collects all ids in the page and checks if all
ids are unique, by asserting the length of the list against it as a set.

Author: gastmaier

tests/test_builders/test_build_html_tocdepth.py

@@ -2,7 +2,7 @@
 
 from __future__ import annotations
 
-from typing import TYPE_CHECKING
+from typing import TYPE_CHECKING, cast
 
 import pytest
 
@@ -12,7 +12,7 @@
 if TYPE_CHECKING:
     from collections.abc import Callable
     from pathlib import Path
-    from xml.etree.ElementTree import ElementTree
+    from xml.etree.ElementTree import Element, ElementTree
 
     from sphinx.testing.util import SphinxTestApp
 
@@ -134,3 +134,17 @@ def test_tocdepth_singlehtml(
 ) -> None:
     app.build()
     check_xpath(cached_etree_parse(app.outdir / 'index.html'), 'index.html', *expect)
+
+
+@pytest.mark.sphinx('singlehtml', testroot='tocdepth')
+@pytest.mark.test_params(shared_result='test_build_html_tocdepth')
+def test_unique_ids_singlehtml(
+    app: SphinxTestApp,
+    cached_etree_parse: Callable[[Path], ElementTree],
+) -> None:
+    app.build()
+    tree = cached_etree_parse(app.outdir / 'index.html')
+    root = cast('Element', tree.getroot())
+
+    ids = [el.attrib['id'] for el in root.findall('.//*[@id]')]
+    assert len(ids) == len(set(ids))