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

registerDirectEvent('onMouseEnter', ['mouseout', 'mouseover']);
  registerDirectEvent('onMouseLeave', ['mouseout', 'mouseover']);
  registerDirectEvent('onPointerEnter', ['pointerout', 'pointerover']);
  registerDirectEvent('onPointerLeave', ['pointerout', 'pointerover']);
}
/**
 * For almost every interaction we care about, there will be both a top-level
 * `mouseover` and `mouseout` event that occurs. Only use `mouseout` so that
 * we do not extract duplicate events. However, moving the mouse into the
 * browser from outside will not fire a `mouseout` event. In this case, we use
 * the `mouseover` top-level event.
 */
function extractEvents(