diff --git a/src/NodeParser/MappedTypeNodeParser.ts b/src/NodeParser/MappedTypeNodeParser.ts
index a1603b7b1..9da5ebe42 100644
--- a/src/NodeParser/MappedTypeNodeParser.ts
+++ b/src/NodeParser/MappedTypeNodeParser.ts
@@ -18,6 +18,7 @@ import { derefAnnotatedType, derefType } from "../Utils/derefType.js";
 import { getKey } from "../Utils/nodeKey.js";
 import { preserveAnnotation } from "../Utils/preserveAnnotation.js";
 import { removeUndefined } from "../Utils/removeUndefined.js";
+import { uniqueTypeArray } from "../Utils/uniqueTypeArray.js";
 
 export class MappedTypeNodeParser implements SubNodeParser {
     public constructor(
@@ -102,8 +103,7 @@ export class MappedTypeNodeParser implements SubNodeParser {
     }
 
     protected getProperties(node: ts.MappedTypeNode, keyListType: UnionType, context: Context): ObjectProperty[] {
-        return keyListType
-            .getFlattenedTypes(derefType)
+        return uniqueTypeArray(keyListType.getFlattenedTypes(derefType))
             .filter((type): type is LiteralType => type instanceof LiteralType)
             .map((type) => [type, this.mapKey(node, type, context)])
             .filter((value): value is [LiteralType, LiteralType] => value[1] instanceof LiteralType)
diff --git a/test/valid-data-type.test.ts b/test/valid-data-type.test.ts
index d91be0ac2..1f2192447 100644
--- a/test/valid-data-type.test.ts
+++ b/test/valid-data-type.test.ts
@@ -102,6 +102,7 @@ describe("valid-data-type", () => {
     it("type-mapped-additional-props", assertValidSchema("type-mapped-additional-props", "MyObject"));
     it("type-mapped-array", assertValidSchema("type-mapped-array", "MyObject"));
     it("type-mapped-union-intersection", assertValidSchema("type-mapped-union-intersection", "MyObject"));
+    it("type-mapped-union-union", assertValidSchema("type-mapped-union-union", "MyType"));
     it("type-mapped-enum", assertValidSchema("type-mapped-enum", "MyObject"));
     it("type-mapped-enum-optional", assertValidSchema("type-mapped-enum-optional", "MyObject"));
     it("type-mapped-enum-null", assertValidSchema("type-mapped-enum-null", "MyObject"));
diff --git a/test/valid-data/type-mapped-union/main.ts b/test/valid-data/type-mapped-union-union/main.ts
similarity index 100%
rename from test/valid-data/type-mapped-union/main.ts
rename to test/valid-data/type-mapped-union-union/main.ts
diff --git a/test/valid-data/type-mapped-union/schema.json b/test/valid-data/type-mapped-union-union/schema.json
similarity index 100%
rename from test/valid-data/type-mapped-union/schema.json
rename to test/valid-data/type-mapped-union-union/schema.json