diff --git a/CHANGES b/CHANGES
index 01a4d3ce2..3c51d6286 100644
--- a/CHANGES
+++ b/CHANGES
@@ -32,6 +32,7 @@ Bugs fixed
 * latex: fix ``\sphinxbfcode`` swallows initial space of argument
 * #3881: LaTeX figure floated to next page sometimes leaves extra vertical
   whitespace
+* #3885: duplicated footnotes raises IndexError
 
 Testing
 --------
diff --git a/sphinx/transforms/__init__.py b/sphinx/transforms/__init__.py
index 0a7db678f..57bdb1059 100644
--- a/sphinx/transforms/__init__.py
+++ b/sphinx/transforms/__init__.py
@@ -289,7 +289,10 @@ class UnreferencedFootnotesDetector(SphinxTransform):
 
     def apply(self):
         for node in self.document.footnotes:
-            if node['names'][0] not in self.document.footnote_refs:
+            if node['names'] == []:
+                # footnote having duplicated number.  It is already warned at parser.
+                pass
+            elif node['names'][0] not in self.document.footnote_refs:
                 logger.warning('Footnote [%s] is not referenced.', node['names'][0],
                                type='ref', subtype='footnote',
                                location=node)