This is happening because of all the running server processes in the background.
So all you need to do is stop them from the terminal.
Quick trick
FOR LINUX
Kill them all by running this on terminal:
pkill -f node
And then restart nodemon.
FOR Windows
1. Go to the task manager
2. Then look for Node.js: Server-side JavaScript
3. Then right click on it and End task from the processes.
Then restart the server. It will work fine.