Fix HMR when asset is duplicated between multiple bundles

packages/core/integration-tests/test/hmr.js

@@ -527,6 +527,21 @@ module.hot.dispose((data) => {
       assert(!reloaded);
     });
 
+    it('should work when an asset is duplicated', async function () {
+      let {reloaded, outputs} = await testHMRClient(
+        'hmr-duplicate',
+        outputs => {
+          assert.deepEqual(outputs, [7]);
+          return {
+            'shared.js': 'exports.a = 5;',
+          };
+        },
+      );
+
+      assert.deepEqual(outputs, [7, 13]);
+      assert(!reloaded);
+    });
+
     it('should bubble to parents if child returns additional parents', async function () {
       let {reloaded, outputs} = await testHMRClient('hmr-parents', outputs => {
         assert.deepEqual(outputs, ['child 2', 'root']);

packages/core/integration-tests/test/integration/hmr-duplicate/a.js

@@ -0,0 +1,3 @@
+const x = require('./shared');
+
+exports.a = x.a + 1;

packages/core/integration-tests/test/integration/hmr-duplicate/b.js

@@ -0,0 +1,3 @@
+const x = require('./shared');
+
+exports.a = x.a + 2;

packages/core/integration-tests/test/integration/hmr-duplicate/index.js

@@ -0,0 +1,14 @@
+let a = import('./a');
+let b = import('./b');
+
+function run() {
+  a.then(function (a) {
+    b.then(function (b) {
+      output(a.a + b.a);
+    });
+  });
+};
+
+module.hot.accept();
+
+run();

packages/core/integration-tests/test/integration/hmr-duplicate/shared.js

@@ -0,0 +1 @@
+exports.a = 2;

packages/runtimes/hmr/src/loaders/hmr-runtime.js

@@ -482,7 +482,11 @@ function hmrApply(bundle /*: ParcelRequire */, asset /*:  HMRAsset */) {
       // $FlowFixMe
       let fn = global.parcelHotUpdate[asset.id];
       modules[asset.id] = [fn, deps];
-    } else if (bundle.parent) {
+    }
+
+    // Always traverse to the parent bundle, even if we already replaced the asset in this bundle.
+    // This is required in case modules are duplicated. We need to ensure all instances have the updated code.
+    if (bundle.parent) {
       hmrApply(bundle.parent, asset);
     }
   }