React version: 0.0.0-f77c7b9d7
Browser: Chrome Version 84.0.4147.105 (Official Build) (64-bit)
OS: Ubuntu 18.04.4 LTS

## Steps To Reproduce

1. move mouse over button

Link to code example:

https://codesandbox.io/s/mouseenter-in-reactnext-ibld3?file=/src/Demo.js

## The current behavior

`onMouseEnter` fires twice (sometimes it doesn't).

![video capture of repro steps](https://i.ibb.co/XzdQF68/react-next-mouseenter-twice.gif)

## The expected behavior

It fires only once. Same repro with `react@16.13.1`: https://codesandbox.io/s/mouseenter-in-react16131-9sr7b?file=/src/Demo.js

render() {
        return <div>Foo</div>;
      }
    }
    const element = <Foo />;
    
    let called = false;
    ReactDOM.render(element, container, () => (called = true));
    expect(called).toEqual(true);
    
    called = false;
    ReactDOM.unstable_batchedUpdates(() => {
      ReactDOM.render(element, container, () => (called = true));
    });
    expect(called).toEqual(true);
  });
  it('should render a component returning strings directly from render', () => {
    const Text = ({value}) => value;
    ReactDOM.render(<Text value="foo" />, container);
    expect(container.textContent).toEqual('foo');
  });
  it('should render a component returning numbers directly from render', () => {
    const Text = ({value}) => value;
    ReactDOM.render(<Text value={10} />, container);
    expect(container.textContent).toEqual('10');
  });
  it('finds the DOM Text node of a string child', () => {
    class Text extends React.Component {