Failing test: Class callback fired multiple times

Happens during a rebase (low priority update followed by high priority
update). The high priority callback gets fired twice.

packages/react-reconciler/src/__tests__/ReactClassSetStateCallback-test.js

@@ -0,0 +1,47 @@
+let React;
+let ReactNoop;
+let Scheduler;
+
+describe('ReactClassSetStateCallback', () => {
+  beforeEach(() => {
+    jest.resetModules();
+
+    React = require('react');
+    ReactNoop = require('react-noop-renderer');
+    Scheduler = require('scheduler');
+  });
+
+  function Text({text}) {
+    Scheduler.unstable_yieldValue(text);
+    return text;
+  }
+
+  test('regression: setState callback (2nd arg) should only fire once, even after a rebase', async () => {
+    let app;
+    class App extends React.Component {
+      state = {step: 0};
+      render() {
+        app = this;
+        return <Text text={this.state.step} />;
+      }
+    }
+
+    const root = ReactNoop.createRoot();
+    await ReactNoop.act(async () => {
+      root.render(<App />);
+    });
+    expect(Scheduler).toHaveYielded([0]);
+
+    await ReactNoop.act(async () => {
+      app.setState({step: 1}, () =>
+        Scheduler.unstable_yieldValue('Callback 1'),
+      );
+      ReactNoop.flushSync(() => {
+        app.setState({step: 2}, () =>
+          Scheduler.unstable_yieldValue('Callback 2'),
+        );
+      });
+    });
+    expect(Scheduler).toHaveYielded([2, 'Callback 2', 2, 'Callback 1']);
+  });
+});