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

ops.push(`Parent:${this.props.step} componentWillUnmount`);
      }
      render() {
        const {step} = this.props;
        return [
          <Child key="a" name={`normal[0]:${step}`} />,
          ReactDOM.createPortal(
            <Child key="b" name={`portal1[0]:${step}`} />,
            portalContainer1,
          ),
          <Child key="c" name={`normal[1]:${step}`} />,
          ReactDOM.createPortal(
            [
              <Child key="d" name={`portal2[0]:${step}`} />,
              <Child key="e" name={`portal2[1]:${step}`} />,
            ],
            portalContainer2,
          ),
        ];
      }
    }
    ReactDOM.render(<Parent step="a" />, container);
    expect(portalContainer1.innerHTML).toBe('<div>portal1[0]:a</div>');
    expect(portalContainer2.innerHTML).toBe(
      '<div>portal2[0]:a</div><div>portal2[1]:a</div>',
    );
    expect(container.innerHTML).toBe(
      '<div>normal[0]:a</div><div>normal[1]:a</div>',
    );
    expect(ops).toEqual([