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

<div key="f">normal[1]</div>,
      ],
      container,
    );
    expect(portalContainer1.innerHTML).toBe(
      '<div>portal1[0]</div><div>portal1[1]</div>',
    );
    expect(portalContainer2.innerHTML).toBe('<div>portal2[0]</div>');
    expect(portalContainer3.innerHTML).toBe('<div>portal3[0]</div>');
    expect(container.innerHTML).toBe(
      '<div>normal[0]</div><div>normal[1]</div>',
    );
    ReactDOM.unmountComponentAtNode(container);
    expect(portalContainer1.innerHTML).toBe('');
    expect(portalContainer2.innerHTML).toBe('');
    expect(portalContainer3.innerHTML).toBe('');
    expect(container.innerHTML).toBe('');
  });
  it('should reconcile portal children', () => {
    const portalContainer = document.createElement('div');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal:1</div>, portalContainer)}</div>,
      container,
    );
    expect(portalContainer.innerHTML).toBe('<div>portal:1</div>');
    expect(container.innerHTML).toBe('<div></div>');
    ReactDOM.render(
      <div>{ReactDOM.createPortal(<div>portal:2</div>, portalContainer)}</div>,
      container,
    );