diff --git a/sphinx/environment/__init__.py b/sphinx/environment/__init__.py
index 4d3935faf..db5a229dc 100644
--- a/sphinx/environment/__init__.py
+++ b/sphinx/environment/__init__.py
@@ -331,8 +331,9 @@ class BuildEnvironment(object):
             self.all_docs[docname] = other.all_docs[docname]
             if docname in other.reread_always:
                 self.reread_always.add(docname)
-            if docname in other.included:
-                self.included.add(docname)
+
+        for docname in other.included:
+            self.included.add(docname)
 
         for version, changes in other.versionchanges.items():
             self.versionchanges.setdefault(version, []).extend(