diff --git a/src/parser/plugins/validate-type.ts b/src/parser/plugins/validate-type.ts
index 1d043919..3c42ba5d 100644
--- a/src/parser/plugins/validate-type.ts
+++ b/src/parser/plugins/validate-type.ts
@@ -1,17 +1,21 @@
 import { getTypeBySource } from '../../type.js';
 import { visitNode } from '../visit.js';
+import { AiScriptSyntaxError } from '../../error.js';
 import type * as Ast from '../../node.js';
 
+function validateTypeParams(node: Ast.Fn): void {
+	const typeParamNames = new Set<string>();
+	for (const typeParam of node.typeParams) {
+		if (typeParamNames.has(typeParam.name)) {
+			throw new AiScriptSyntaxError(`type parameter name ${typeParam.name} is duplicate`, node.loc.start);
+		}
+		typeParamNames.add(typeParam.name);
+	}
+}
+
 function collectTypeParams(node: Ast.Node, ancestors: Ast.Node[]): Ast.TypeParam[] {
 	const items = [];
 	if (node.type === 'fn') {
-		const typeParamNames = new Set<string>();
-		for (const typeParam of node.typeParams) {
-			if (typeParamNames.has(typeParam.name)) {
-				throw new Error(`type parameter name ${typeParam.name} is duplicate`);
-			}
-			typeParamNames.add(typeParam.name);
-		}
 		items.push(...node.typeParams);
 	}
 	for (let i = ancestors.length - 1; i >= 0; i--) {
@@ -32,6 +36,7 @@ function validateNode(node: Ast.Node, ancestors: Ast.Node[]): Ast.Node {
 			break;
 		}
 		case 'fn': {
+			validateTypeParams(node);
 			for (const param of node.params) {
 				if (param.argType != null) {
 					getTypeBySource(param.argType, collectTypeParams(node, ancestors));
diff --git a/test/types.ts b/test/types.ts
index d876e00a..63ad5db2 100644
--- a/test/types.ts
+++ b/test/types.ts
@@ -1,8 +1,8 @@
 import * as assert from 'assert';
-import { describe, test } from 'vitest';
+import { describe, expect, test } from 'vitest';
 import { utils } from '../src';
 import { NUM, STR, NULL, ARR, OBJ, BOOL, TRUE, FALSE, ERROR ,FN_NATIVE } from '../src/interpreter/value';
-import { AiScriptRuntimeError } from '../src/error';
+import { AiScriptRuntimeError, AiScriptSyntaxError } from '../src/error';
 import { exe, getMeta, eq } from './testutils';
 
 describe('function types', () => {
@@ -98,9 +98,15 @@ describe('generics', () => {
 		});
 
 		test.concurrent('duplicate', async () => {
-			await assert.rejects(() => exe(`
+			await expect(() => exe(`
 			@f<T, T>(v: T) {}
-			`));
+			`)).rejects.toThrow(AiScriptSyntaxError);
+		});
+
+		test.concurrent('duplicate (no param and ret types)', async () => {
+			await expect(() => exe(`
+			@f<T, T>() {}
+			`)).rejects.toThrow(AiScriptSyntaxError);
 		});
 
 		test.concurrent('empty', async () => {
diff --git a/unreleased/fix-duplicate-type-parameter-name-error.md b/unreleased/fix-duplicate-type-parameter-name-error.md
new file mode 100644
index 00000000..362fcea5
--- /dev/null
+++ b/unreleased/fix-duplicate-type-parameter-name-error.md
@@ -0,0 +1,2 @@
+- Fix: 関数の型引数の名前が重複した場合に適切なエラーが発生しない問題を修正
+- Fix: 関数の引数や返り値に型注釈が無く、型引数の名前が重複した場合にエラーが発生しない問題を修正