diff --git a/CHANGES b/CHANGES
index 63cc1984d..8074e8a9d 100644
--- a/CHANGES
+++ b/CHANGES
@@ -112,6 +112,8 @@ Features added
 Release 1.0.8 (in development)
 ==============================
 
+* #704: Fix image file duplication bug.
+
 * #677: Fix parsing of multiple signatures in C++ domain.
 
 * #637: Ignore Emacs lock files when looking for source files.
diff --git a/sphinx/util/__init__.py b/sphinx/util/__init__.py
index 0721118e7..0d67da82c 100644
--- a/sphinx/util/__init__.py
+++ b/sphinx/util/__init__.py
@@ -112,11 +112,11 @@ class FilenameUniqDict(dict):
         return uniquename
 
     def purge_doc(self, docname):
-        for filename, (docs, _) in self.items():
+        for filename, (docs, unique) in self.items():
             docs.discard(docname)
             if not docs:
                 del self[filename]
-                self._existing.discard(filename)
+                self._existing.discard(unique)
 
     def __getstate__(self):
         return self._existing