The bug

mouseenter handler gets fired twice, in a component ReactDOM.render-ed in another component.

ReactDOM.render(<AnotherComponent />, hostEl, () => {
  ReactDOM.render(<AComponentMouseEnterWronglyFiredTwice />, hostEl.childNodes[1]);
});

function AComponentMouseEnterWronglyFiredTwice() {
     return (<>
        <div
            onMouseEnter={() => {console.log('You may see this twice');}}
        >
            Move mouse TO here
        </div>
        <div>Move mouse FROM here</div>
     </>)
}

The bug issue #16763
The reproduction demo https://codesandbox.io/s/hungry-field-kgkli

The cause

For any event happened in ReactDOM.render-ed component in another component.
In function handleTopLevel(bookKeeping) {...}, we'll get two ancestors (at least) for bookKeeping.

It's ok for most of the events, but mouseenter is different.
mouseenter is extracted out from mouseout/mouseover.

When mouseout gets dealt twice in handleTopLevel in the situation above, mouseenter gets fired twice on the same node.

The fix

EnterLeaveEventPlugin.js

extractEvents(...) {
    ...
    if (isOutEvent && from && nativeEventTarget !== fromNode) {
      return [leave];
    }
    return [leave, enter];
}