diff --git a/oi/CodeGen.cpp b/oi/CodeGen.cpp
index 9f41c12..9614a1f 100644
--- a/oi/CodeGen.cpp
+++ b/oi/CodeGen.cpp
@@ -213,9 +213,13 @@ template <typename T>
 struct NameProvider {};
 )";
 
+  // TODO: stop types being duplicated at this point and remove this check
+  std::unordered_set<std::string_view> emittedTypes;
   for (const Type& t : typeGraph.finalTypes) {
     if (dynamic_cast<const Typedef*>(&t))
       continue;
+    if (!emittedTypes.emplace(t.name()).second)
+      continue;
 
     code += "template <> struct NameProvider<";
     code += t.name();