diff --git a/src/parser/syntaxes/expressions.ts b/src/parser/syntaxes/expressions.ts
index f0a96cb5..fcc59ff0 100644
--- a/src/parser/syntaxes/expressions.ts
+++ b/src/parser/syntaxes/expressions.ts
@@ -593,6 +593,9 @@ function parseObject(s: ITokenStream, isStatic: boolean): Ast.Obj {
 	const map = new Map<string, Ast.Expression>();
 	while (!s.is(TokenKind.CloseBrace)) {
 		const k = parseObjectKey(s);
+		if (map.has(k)) {
+			throw new AiScriptSyntaxError(`Key ${k} is duplicated.`, s.getPos());
+		}
 		s.next();
 
 		s.expect(TokenKind.Colon);
diff --git a/test/literals.ts b/test/literals.ts
index cb747b7c..330c82bb 100644
--- a/test/literals.ts
+++ b/test/literals.ts
@@ -251,6 +251,12 @@ describe('literal', () => {
 		`)).rejects.toThrow(AiScriptSyntaxError);
 	})
 
+	test.concurrent('obj (duplicate key)', async () => {
+		await expect(() => exe(`
+			<: { hoge: 1, hoge: 2 }
+		`)).rejects.toThrow(AiScriptSyntaxError);
+	});
+
 	test.concurrent('obj (invalid key)', async () => {
 		assert.rejects(() => exe(`
 		<: {
diff --git a/unreleased/dupe-key-to-syntax-error.md b/unreleased/dupe-key-to-syntax-error.md
new file mode 100644
index 00000000..1fbf5d88
--- /dev/null
+++ b/unreleased/dupe-key-to-syntax-error.md
@@ -0,0 +1 @@
+- Fix: オブジェクトリテラルでキーの重複がエラーになるように修正