diff --git a/src/engine/engine.test.ts b/src/engine/engine.test.ts
index 29f15e1..febef3a 100644
--- a/src/engine/engine.test.ts
+++ b/src/engine/engine.test.ts
@@ -335,6 +335,13 @@ describe("extension API", () => {
     eng.addUnit({ id: "horse", phrases: "horse, horses, hrs", baseUnitId: "meter", format: "hrs", ratio: 2.4 });
     expect(calc("2 horses in meters")).toBe("4.8 m");
   });
+  it("addUnit throws on id collision instead of silently corrupting unitsById", () => {
+    expect(() =>
+      eng.addUnit({ id: "horse", phrases: "nag", baseUnitId: "meter", format: "nag", ratio: 99 })
+    ).toThrow(/horse/);
+    // the original registration must survive the failed collision
+    expect(calc("2 horses in meters")).toBe("4.8 m");
+  });
   it("addFunction (zum from Sample.js)", () => {
     eng.addFunction({ id: "zum", phrases: "zum" }, (values) => ({ double: values[0].double + values[1].double }));
     expect(calc("zum(2;3)")).toBe("5");
diff --git a/src/engine/index.ts b/src/engine/index.ts
index e507dd9..5d6e82e 100644
--- a/src/engine/index.ts
+++ b/src/engine/index.ts
@@ -218,6 +218,7 @@ export class SumEngine {
   addUnit(spec: ExtensionUnitSpec): void {
     const base = this.reg.unitsById.get(spec.baseUnitId) ?? this.currencyBase(spec.baseUnitId);
     if (!base) throw new Error(`numi.addUnit: unknown baseUnitId ${spec.baseUnitId}`);
+    if (this.reg.unitsById.has(spec.id)) throw new Error(`numi.addUnit: id "${spec.id}" is already registered`);
     const unit: Unit = {
       id: spec.id,
       dimension: base.dimension,