diff --git a/CHANGES b/CHANGES
index 23a4d0f3b..cd2acfd0c 100644
--- a/CHANGES
+++ b/CHANGES
@@ -166,6 +166,7 @@ Features added
 * C++: add ``cpp:struct`` to complement ``cpp:class``.
 * #1341 the HTML search considers words that contain a search term of length
   three or longer a match.
+* #4611: epub: Show warning for duplicated ToC entries
 
 Bugs fixed
 ----------
diff --git a/sphinx/builders/_epub_base.py b/sphinx/builders/_epub_base.py
index 7038f45f6..5516fdaf8 100644
--- a/sphinx/builders/_epub_base.py
+++ b/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)
diff --git a/tests/roots/test-root/index.txt b/tests/roots/test-root/index.txt
index ce0338cf7..27f90f357 100644
--- a/tests/roots/test-root/index.txt
+++ b/tests/roots/test-root/index.txt
@@ -24,7 +24,6 @@ Contents:
    math
    autodoc
    extensions
-   extensions
    footnote
    lists
    otherext
diff --git a/tests/roots/test-toctree-duplicated/conf.py b/tests/roots/test-toctree-duplicated/conf.py
new file mode 100644
index 000000000..e69de29bb
diff --git a/tests/roots/test-toctree-duplicated/foo.rst b/tests/roots/test-toctree-duplicated/foo.rst
new file mode 100644
index 000000000..f23d4ce05
--- /dev/null
+++ b/tests/roots/test-toctree-duplicated/foo.rst
@@ -0,0 +1,2 @@
+foo
+===
diff --git a/tests/roots/test-toctree-duplicated/index.rst b/tests/roots/test-toctree-duplicated/index.rst
new file mode 100644
index 000000000..38a8c44c1
--- /dev/null
+++ b/tests/roots/test-toctree-duplicated/index.rst
@@ -0,0 +1,7 @@
+test-toctree-duplicated
+=======================
+
+.. toctree::
+
+   foo
+   foo
diff --git a/tests/test_build_epub.py b/tests/test_build_epub.py
index 86e40b65d..337363832 100644
--- a/tests/test_build_epub.py
+++ b/tests/test_build_epub.py
@@ -367,6 +367,12 @@ def test_html_download_role(app, status, warning):
             '/_static/sphinxheader.png]</span></p></li>' in content)
 
 
+@pytest.mark.sphinx('epub', testroot='toctree-duplicated')
+def test_duplicated_toctree_entry(app, status, warning):
+    app.build()
+    assert 'WARNING: duplicated ToC entry found: foo.xhtml' in warning.getvalue()
+
+
 @pytest.mark.skipif('DO_EPUBCHECK' not in os.environ,
                     reason='Skipped because DO_EPUBCHECK is not set')
 @pytest.mark.sphinx('epub')