Fix #4611: epub: Show warning for duplicated ToC entries

sphinx/builders/_epub_base.py

@@ -35,7 +35,7 @@ except ImportError:
 
 if False:
     # For type annotation
-    from typing import Any, Dict, List, Tuple  # NOQA
+    from typing import Any, Dict, List, Set, Tuple  # NOQA
     from sphinx.application import Sphinx  # NOQA
 
 
@@ -213,6 +213,15 @@ class EpubBuilder(StandaloneHTMLBuilder):
                 result = self.get_refnodes(elem, result)
         return result
 
+    def check_refnodes(self, nodes):
+        # type: (List[Dict[str, Any]]) -> None
+        appeared = set()  # type: Set[str]
+        for node in nodes:
+            if node['refuri'] in appeared:
+                logger.warning(__('duplicated ToC entry found: %s'), node['refuri'])
+            else:
+                appeared.add(node['refuri'])
+
     def get_toc(self):
         # type: () -> None
         """Get the total table of contents, containing the master_doc
@@ -726,6 +735,7 @@ class EpubBuilder(StandaloneHTMLBuilder):
         else:
             # 'includehidden'
             refnodes = self.refnodes
+        self.check_refnodes(refnodes)
         navpoints = self.build_navpoints(refnodes)
         level = max(item['level'] for item in self.refnodes)
         level = min(level, self.config.epub_tocdepth)