diff --git a/CHANGES b/CHANGES
index 7756f7096..6f7367919 100644
--- a/CHANGES
+++ b/CHANGES
@@ -25,6 +25,7 @@ Bugs fixed
 * #5810: LaTeX: sphinxVerbatim requires explicit "hllines" set-up since 1.6.6
   (refs: #1238)
 * #5636: C++, fix parsing of floating point literals.
+* #5496 (again): C++, fix assertion in partial builds with duplicates.
 
 Testing
 --------
diff --git a/sphinx/domains/cpp.py b/sphinx/domains/cpp.py
index 7a84c3ba6..62767c15e 100644
--- a/sphinx/domains/cpp.py
+++ b/sphinx/domains/cpp.py
@@ -3615,6 +3615,7 @@ class SymbolLookupResult(object):
 
 class Symbol(object):
     debug_lookup = False
+    debug_show_tree = False
 
     def _assert_invariants(self):
         # type: () -> None
@@ -4037,11 +4038,10 @@ class Symbol(object):
             print("      #noDecl:  ", len(noDecl))
             print("      #withDecl:", len(withDecl))
             print("      #dupDecl: ", len(dupDecl))
-        if len(dupDecl) > 0:
-            assert len(withDecl) > 0
-        # assert len(noDecl) <= 1  # we should fill in symbols when they are there
-        # TODO: enable assertion when we at some point find out how to do cleanup
         # With partial builds we may start with a large symbol tree stripped of declarations.
+        # Essentially any combination of noDecl, withDecl, and dupDecls seems possible.
+        # TODO: make partial builds fully work. What should happen when the primary symbol gets
+        #  deleted, and other duplicates exist? The full document should probably be rebuild.
 
         # First check if one of those with a declaration matches.
         # If it's a function, we need to compare IDs,
@@ -4294,6 +4294,8 @@ class Symbol(object):
                 res.append(text_type(self.templateArgs))
             if self.declaration:
                 res.append(": ")
+                if self.isRedeclaration:
+                    res.append('!!duplicate!! ')
                 res.append(text_type(self.declaration))
         if self.docname:
             res.append('\t(')
@@ -6691,18 +6693,30 @@ class CPPDomain(Domain):
 
     def clear_doc(self, docname):
         # type: (unicode) -> None
+        if Symbol.debug_show_tree:
+            print("clear_doc:", docname)
+            print("\tbefore:")
+            print(self.data['root_symbol'].dump(1))
+            print("\tbefore end")
+
         rootSymbol = self.data['root_symbol']
         rootSymbol.clear_doc(docname)
+
+        if Symbol.debug_show_tree:
+            print("\tafter:")
+            print(self.data['root_symbol'].dump(1))
+            print("\tafter end")
+            print("clear_doc end:", docname)
         for name, nDocname in list(self.data['names'].items()):
             if nDocname == docname:
                 del self.data['names'][name]
 
     def process_doc(self, env, docname, document):
         # type: (BuildEnvironment, unicode, nodes.Node) -> None
-        # just for debugging
-        # print("process_doc:", docname)
-        # print(self.data['root_symbol'].dump(0))
-        pass
+        if Symbol.debug_show_tree:
+            print("process_doc:", docname)
+            print(self.data['root_symbol'].dump(0))
+            print("process_doc end:", docname)
 
     def process_field_xref(self, pnode):
         # type: (nodes.Node) -> None
@@ -6710,11 +6724,15 @@ class CPPDomain(Domain):
 
     def merge_domaindata(self, docnames, otherdata):
         # type: (List[unicode], Dict) -> None
-        # print("merge_domaindata:")
-        # print("self")
-        # print(self.data['root_symbol'].dump(0))
-        # print("other:")
-        # print(otherdata['root_symbol'].dump(0))
+        if Symbol.debug_show_tree:
+            print("merge_domaindata:")
+            print("\tself:")
+            print(self.data['root_symbol'].dump(1))
+            print("\tself end")
+            print("\tother:")
+            print(otherdata['root_symbol'].dump(1))
+            print("\tother end")
+            print("merge_domaindata end")
 
         self.data['root_symbol'].merge_with(otherdata['root_symbol'],
                                             docnames, self.env)