Component sometimes fires event handlers twice

[patrick-radulian]

Do you want to request a feature or report a bug?
I want to report a bug.

What is the current behavior?
Event handlers on elements are fired twice, when hovering from one element onto the next.

If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:
https://codesandbox.io/s/quizzical-mountain-9s48c

What is the expected behavior?
Event handlers should only fire once.

Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?
React 16.8.4
ReactDOM 16.8.4
Chrome 76.0.3809.132
Firefox 69.0 (64-Bit)
Edge 44.18362.267.0

[yuanoook]

After testing it again and again, I found how to reproduce it:

Step 1, render a new component inside a rendered component;
Step 2, inside the new component, mouse enter a node from its sibling.

Current result: onMouseEnter is called more than once.
Demo: https://codesandbox.io/s/hungry-field-kgkli

@bvaughn We may change the issue title because it's not related with dangerouslySetInnerHTML.

I've made a test and fixture for this: yuanoook@71dcd9e

Hope the information can be helpful. I still have no idea how to fix it.

If there's no one working on this, I would like to pick this up.

There's a similar issue #10906
@gaearon Do you have any advice?

[bvaughn]

If there's no one working on this, I would like to pick this up.

Go for it, @yuanoook!

[souravbadami]

Let's close this? I see the fix was merged in #16928.