This new check is in preparation for an upcoming feature that lets you preserve state between unmounts. For example, when switching a between routes or changing a top-level tab. The check simulates unmounting and immediately remounting (with state/DOM preserved), which is why effects run twice. It simulates what happens when you switch away to another tab and then back, like a stress-test for this scenario. Since in most cases, switching to another tab and then immediately switching back shouldn’t be observable behavior. You might fire an extra impression log in development, but from the user perspective the behavior should be the same. And if it’s not, you’ll see the same result as if the user actually switched tabs there and back. So it doesn’t just “run the code twice” but it simulates what happens when a tree remounts with existing state. Most components are already resilient to this, but the feature is hard to adopt if this resiliency isn’t being tested. This check helps find the remaining cases where remounting wouldn’t be safe, early.

Hope this clarifies it a bit!

https://reactjs.org/blog/2022/03/08/react-18-upgrade-guide.h...