Remap: duplicate targets (#3025)

#2878 (comment)

<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit

* **Bug Fixes**
* Remap now detects and rejects duplicate target keys in mappings to
prevent key collisions.

* **Tests**
* Added a runtime test asserting an error is thrown for duplicate
mapping targets.

* **Documentation**
* CHANGELOG updated to document the remap duplicate-target fix (v2.1.0).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

Author: RobinTail

zod-plugin/CHANGELOG.md

@@ -2,6 +2,10 @@
 
 ## Version 2
 
+### v2.1.0
+
+- `ZodObject::remap()` now throws an `Error` if duplicate target keys found in its argument.
+
 ### v2.0.0
 
 - Restricting the supported Node.js versions: `^20.19.0 || ^22.12.0 || ^24.0.0`.

zod-plugin/src/remap.ts

@@ -27,13 +27,20 @@ type Mapper = <T extends Record<string, unknown>>(
   subject: T,
 ) => { [P in string | keyof T]: T[keyof T] };
 
+const makeTransformer = (mapping: Partial<Record<string, string>>) => {
+  const clean = R.reject(R.isNil, mapping); // rejecting undefined
+  const targets = Object.values(clean);
+  if (new Set(targets).size !== targets.length)
+    throw new Error("remap(): duplicate target keys", { cause: mapping });
+  return R.renameKeys(clean);
+};
+
 /** Used by runtime (bound) */
 export const remap = function (
   this: z.ZodObject,
-  tool: Record<string, string> | Mapper,
+  tool: Parameters<typeof makeTransformer>[0] | Mapper,
 ) {
-  const transformer =
-    typeof tool === "function" ? tool : R.renameKeys(R.reject(R.isNil, tool)); // rejecting undefined
+  const transformer = typeof tool === "function" ? tool : makeTransformer(tool);
   const nextShape = transformer(
     R.map(R.invoker(0, "clone"), this._zod.def.shape), // immutable, changed from R.clone due to failure
   );

zod-plugin/tests/runtime.spec.ts

@@ -117,6 +117,13 @@ describe("Zod Runtime Plugin", () => {
       },
     );
 
+    test("throws when multiple source keys map to the same target", () => {
+      const schema = z.object({ a: z.number(), b: z.string() });
+      expect(() => schema.remap({ a: "x", b: "x" })).toThrowError(
+        /duplicate target keys/,
+      );
+    });
+
     test("should support a mapping function", () => {
       const schema = z.object({ user_id: z.string(), name: z.string() });
       const mappedSchema = schema.remap((shape) => camelize(shape, true));