Something is already when using Create-react-app running on port 3000

Problem:

Today, when using Create-react-app to build the react application, I opened two react apps, and then NPM start, it appeared something is already running on Port 3000

Workaround

I have encountered this problem before, but did not record, I try to think, first NPM start port=3001, the result is still wrong.
The correct way to open it should be port=3001 NPM start (this port is as good as 3000).

Sum up, lest you look good later. Also share to the small partners who have encountered this problem yo

Something is already when using Create-react-app running on port 3000