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 !this.props.flag
          ? [<div key="a" />]
          : [<span key="b" />, <div key="a" />];
      }
    }
    const myNodeA = ReactDOM.render(<MyNode />, container);
    const a = ReactDOM.findDOMNode(myNodeA);
    expect(a.tagName).toBe('DIV');
    const myNodeB = ReactDOM.render(<MyNode flag={true} />, container);
    expect(myNodeA === myNodeB).toBe(true);
    const b = ReactDOM.findDOMNode(myNodeB);
    expect(b.tagName).toBe('SPAN');
  });
  it('should bubble events from the portal to the parent', () => {
    const portalContainer = document.createElement('div');
    document.body.appendChild(portalContainer);
    try {
      const ops = [];
      let portal = null;
      ReactDOM.render(
        <div onClick={() => ops.push('parent clicked')}>
          {ReactDOM.createPortal(
            <div
              onClick={() => ops.push('portal clicked')}
              ref={n => (portal = n)}>