diff --git a/upb/def.c b/upb/def.c
index 363d8bb960..88237ab35c 100644
--- a/upb/def.c
+++ b/upb/def.c
@@ -2397,6 +2397,12 @@ static int count_bits_debug(uint64_t x) {
   return n;
 }
 
+static int compare_int32(const void* a_ptr, const void* b_ptr) {
+  int32_t a = *(int32_t*)a_ptr;
+  int32_t b = *(int32_t*)b_ptr;
+  return ((a) < (b) ? -1 : ((a) == (b) ? 0 : 1));
+}
+
 upb_MiniTable_Enum* create_enumlayout(symtab_addctx* ctx,
                                       const upb_EnumDef* e) {
   int n = 0;
@@ -2427,6 +2433,17 @@ upb_MiniTable_Enum* create_enumlayout(symtab_addctx* ctx,
     UPB_ASSERT(p == values + n);
   }
 
+  // Enums can have duplicate values; we must sort+uniq them.
+  qsort(values, n, sizeof(*values), &compare_int32);
+
+  int dst = 0;
+  for (int i = 0; i < n; dst++) {
+    int32_t val = values[i];
+    while (i < n && values[i] == val) i++;  // Skip duplicates.
+    values[dst] = val;
+  }
+  n = dst;
+
   UPB_ASSERT(upb_inttable_count(&e->iton) == n + count_bits_debug(mask));
 
   upb_MiniTable_Enum* layout = symtab_alloc(ctx, sizeof(*layout));
@@ -2510,7 +2527,7 @@ static void create_enumdef(
     if (ctx->layout) {
       UPB_ASSERT(ctx->enum_count < ctx->layout->enum_count);
       e->layout = ctx->layout->enums[ctx->enum_count++];
-      UPB_ASSERT(n ==
+      UPB_ASSERT(upb_inttable_count(&e->iton) ==
                  e->layout->value_count + count_bits_debug(e->layout->mask));
     } else {
       e->layout = create_enumlayout(ctx, e);
diff --git a/upb/util/def_to_proto_test.proto b/upb/util/def_to_proto_test.proto
index c278748f88..2e4cac5f76 100644
--- a/upb/util/def_to_proto_test.proto
+++ b/upb/util/def_to_proto_test.proto
@@ -69,6 +69,26 @@ enum Enum {
   NEGATIVE_ONE = -1;
 }
 
+enum EnumUpper32Value {
+  UPPER32_VALUE = 40;
+}
+
+enum HasDuplicateValues {
+  option allow_alias = true;
+  A = 0;
+  B = 1;
+  C = 120;
+  D = 130;
+
+  G = 120;
+  F = 1;
+  E = 0;
+  H = 121;
+  I = 121;
+  J = 121;
+  K = 121;
+}
+
 service Service {
   rpc Bar(Message) returns (Message);
 }
diff --git a/upbc/protoc-gen-upb.cc b/upbc/protoc-gen-upb.cc
index ff4a5028f1..da1e7f2de0 100644
--- a/upbc/protoc-gen-upb.cc
+++ b/upbc/protoc-gen-upb.cc
@@ -95,6 +95,17 @@ std::vector<const protobuf::EnumDescriptor*> SortedEnums(
   return enums;
 }
 
+std::vector<int32_t> SortedUniqueEnumNumbers(
+    const protobuf::EnumDescriptor* e) {
+  std::vector<int32_t> values;
+  for (int i = 0; i < e->value_count(); i++) {
+    values.push_back(e->value(i)->number());
+  }
+  std::sort(values.begin(), values.end());
+  std::unique(values.begin(), values.end());
+  return values;
+}
+
 void AddMessages(const protobuf::Descriptor* message,
                  std::vector<const protobuf::Descriptor*>* messages) {
   messages->push_back(message);
@@ -1301,8 +1312,7 @@ int WriteEnums(const protobuf::FileDescriptor* file, Output& output) {
   for (const auto* e : this_file_enums) {
     uint64_t mask = 0;
     absl::flat_hash_set<int32_t> values;
-    for (int i = 0; i < e->value_count(); i++) {
-      int32_t number = e->value(i)->number();
+    for (auto number : SortedUniqueEnumNumbers(e)) {
       if (static_cast<uint32_t>(number) < 64) {
         mask |= 1 << number;
       } else {