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 onClick="woops" />;
      }
    }
    expect(() => ReactDOM.render(<Example />, container)).toErrorDev(
      'Expected `onClick` listener to be a function, instead got a value of `string` type.\n' +
        '    in div (at **)\n' +
        '    in Example (at **)',
    );
  });
  it('should warn with a special message for `false` event listeners', () => {
    class Example extends React.Component {