diff --git a/codegen/codegen.go b/codegen/codegen.go
index 1b29e27..f658e8f 100644
--- a/codegen/codegen.go
+++ b/codegen/codegen.go
@@ -215,12 +215,17 @@ func validateSlotKeys(slots map[string]string) error {
 
 func orderedSlotEntries(slots map[string]string) []slotEntry {
 	entries := make([]slotEntry, 0, len(slots))
+	seenTags := make(map[string]struct{}, len(slots))
 
 	for _, slot := range canonicalSlotOrder {
 		tag, ok := slots[slot]
 		if !ok {
 			continue
 		}
+		if _, seen := seenTags[tag]; seen {
+			continue
+		}
+		seenTags[tag] = struct{}{}
 		entries = append(entries, slotEntry{Slot: slot, Tag: tag})
 	}
 	return entries
diff --git a/codegen/codegen_test.go b/codegen/codegen_test.go
index 7519285..f0702c1 100644
--- a/codegen/codegen_test.go
+++ b/codegen/codegen_test.go
@@ -67,6 +67,21 @@ func TestGenerateBundle_Good(t *testing.T) {
 	assert.True(t, h < c && c < f, "expected canonical HLCRF order in generated bundle")
 }
 
+func TestGenerateBundle_DeduplicatesTags(t *testing.T) {
+	slots := map[string]string{
+		"H": "nav-bar",
+		"C": "nav-bar",
+		"F": "page-footer",
+	}
+
+	js, err := GenerateBundle(slots)
+	require.NoError(t, err)
+
+	assert.Equal(t, 2, strings.Count(js, "extends HTMLElement"))
+	assert.Equal(t, 1, strings.Count(js, "class NavBar extends HTMLElement"))
+	assert.Equal(t, 1, strings.Count(js, "class PageFooter extends HTMLElement"))
+}
+
 func TestGenerateBundle_Bad_InvalidSlotKey(t *testing.T) {
 	slots := map[string]string{
 		"H": "nav-bar",
@@ -98,6 +113,21 @@ func TestGenerateTypeDefinitions_Good(t *testing.T) {
 	assert.Contains(t, dts, "export {};")
 }
 
+func TestGenerateTypeDefinitions_DeduplicatesTags(t *testing.T) {
+	slots := map[string]string{
+		"H": "nav-bar",
+		"C": "nav-bar",
+		"F": "page-footer",
+	}
+
+	dts, err := GenerateTypeDefinitions(slots)
+	require.NoError(t, err)
+
+	assert.Equal(t, 2, strings.Count(dts, "extends HTMLElement"))
+	assert.Equal(t, 1, strings.Count(dts, `"nav-bar": NavBar;`))
+	assert.Equal(t, 1, strings.Count(dts, `"page-footer": PageFooter;`))
+}
+
 func TestGenerateTypeDefinitions_SkipsInvalidTags(t *testing.T) {
 	slots := map[string]string{
 		"H": "nav-bar",