diff --git a/src/reactivity/track.ts b/src/reactivity/track.ts
index 8199ca7..af49749 100644
--- a/src/reactivity/track.ts
+++ b/src/reactivity/track.ts
@@ -39,6 +39,9 @@ const _isDev = isDev();
 // raw ESM it is undefined, so we fall back to "dev". Only used to enrich the
 // multi-instance dev warning.
 declare const __SIBU_VERSION__: string | undefined;
+// The `__SIBU_VERSION__` branch only runs when a bundler inlined the define; the
+// source test runner always takes the "dev" fallback, so that side is excluded.
+/* v8 ignore next */
 const _runtimeVersion = typeof __SIBU_VERSION__ !== "undefined" ? __SIBU_VERSION__ : "dev";
 
 interface ReactiveApi {
diff --git a/tests/duplicate-instance-source.test.ts b/tests/duplicate-instance-source.test.ts
new file mode 100644
index 0000000..db395b1
--- /dev/null
+++ b/tests/duplicate-instance-source.test.ts
@@ -0,0 +1,68 @@
+import { afterEach, beforeEach, describe, expect, test, vi } from "vitest";
+
+// ---------------------------------------------------------------------------
+// Covers the duplicate-instance DETECTION path on the real source modules.
+//
+// `tests/duplicate-instance.test.ts` proves cross-instance behavior by bundling
+// the core and evaluating it twice — but that runs a bundled copy, so coverage
+// instrumentation never credits the source module's "a duplicate loaded" branch
+// (the in-process module evaluates exactly once, always taking the first-copy
+// path). Here we exercise that branch directly: pre-seed the `globalThis`
+// registry as if a first copy had already published its API, then freshly
+// import the source module so its resolver takes the duplicate path.
+// ---------------------------------------------------------------------------
+
+const REACTIVE_KEY = Symbol.for("sibujs.reactive.v1");
+const BATCH_KEY = Symbol.for("sibujs.reactive.batch.v1");
+
+type Registry = Record<symbol, unknown>;
+
+describe("duplicate-instance detection on the source modules", () => {
+  beforeEach(() => {
+    vi.resetModules();
+  });
+
+  afterEach(() => {
+    delete (globalThis as Registry)[REACTIVE_KEY];
+    delete (globalThis as Registry)[BATCH_KEY];
+    vi.restoreAllMocks();
+  });
+
+  test("track.ts delegates to the first copy and dev-warns once on a duplicate load", async () => {
+    // Simulate a first copy of the reactive runtime having published its API.
+    (globalThis as Registry)[REACTIVE_KEY] = { version: "0.0.0-first", __dupWarned: false };
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    await import("../src/reactivity/track");
+
+    expect(warn).toHaveBeenCalledTimes(1);
+    expect(String(warn.mock.calls[0][0])).toContain("Multiple instances of the reactive runtime");
+    // The registry object is stamped so a third copy stays quiet.
+    expect((globalThis as Record<symbol, { __dupWarned?: boolean }>)[REACTIVE_KEY].__dupWarned).toBe(true);
+  });
+
+  test("track.ts does not warn again when the first copy already warned", async () => {
+    (globalThis as Registry)[REACTIVE_KEY] = { version: "0.0.0-first", __dupWarned: true };
+    const warn = vi.spyOn(console, "warn").mockImplementation(() => {});
+
+    await import("../src/reactivity/track");
+
+    expect(warn).not.toHaveBeenCalled();
+  });
+
+  test("batch.ts re-exports the first copy's functions on a duplicate load", async () => {
+    const firstBatch = {
+      batch: <T>(fn: () => T): T => fn(),
+      enqueueBatchedSignal: () => false,
+      isBatching: () => false,
+    };
+    (globalThis as Registry)[BATCH_KEY] = firstBatch;
+
+    const mod = await import("../src/reactivity/batch");
+
+    // The duplicate copy delegates to the first copy's functions verbatim.
+    expect(mod.batch).toBe(firstBatch.batch);
+    expect(mod.isBatching).toBe(firstBatch.isBatching);
+    expect(mod.enqueueBatchedSignal).toBe(firstBatch.enqueueBatchedSignal);
+  });
+});