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

/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import type {AnyNativeEvent} from '../PluginModuleType';
import type {DOMEventName} from '../DOMEventNames';
import type {DispatchQueue} from '../DOMPluginEventSystem';
import type {EventSystemFlags} from '../EventSystemFlags';
import {registerDirectEvent} from '../EventRegistry';
import {IS_REPLAYED} from 'react-dom/src/events/EventSystemFlags';
import {
  SyntheticEvent,
  MouseEventInterface,
  PointerEventInterface,
} from '../SyntheticEvent';
import {
  getClosestInstanceFromNode,
  getNodeFromInstance,
} from '../../client/ReactDOMComponentTree';
import {accumulateEnterLeaveTwoPhaseListeners} from '../DOMPluginEventSystem';
import {HostComponent, HostText} from 'react-reconciler/src/ReactWorkTags';
import {getNearestMountedFiber} from 'react-reconciler/src/ReactFiberTreeReflection';
function registerEvents() {