--- a/ipc/ipdl/ipdl/lower.py
+++ b/ipc/ipdl/ipdl/lower.py
@@ -1386,18 +1386,20 @@ class _GenerateProtocolCode(ipdl.ast.Vis
         for inc in builtinHeaderIncludes:
             self.visitBuiltinCxxInclude(inc)
 
         # Compute the set of includes we need for declared structure/union
         # classes for this protocol.
         typesToIncludes = {}
         for using in tu.using:
             typestr = str(using.type.spec)
-            assert typestr not in typesToIncludes
-            typesToIncludes[typestr] = using.header
+            if typestr not in typesToIncludes:
+                typesToIncludes[typestr] = using.header
+            else:
+                assert typesToIncludes[typestr] == using.header
 
         aggregateTypeIncludes = set()
         for su in tu.structsAndUnions:
             typedeps = _ComputeTypeDeps(su.decl.type, True)
             if isinstance(su, ipdl.ast.StructDecl):
                 for f in su.fields:
                     f.ipdltype.accept(typedeps)
             elif isinstance(su, ipdl.ast.UnionDecl):