severless not working when I added app.listen() in Express.js

Serverless Framework
1

When I added any port in my express app
app.listen(port, async () => {
//code
});

I will got error when i have more than one request to “express server”

Error: listen EADDRINUSE: address already in use :::port

if i remove that. it work well.

So i was make some test by added some log into app.listen

app.listen("4000",  () => {
  console.log(`Running at localhost:4000`);
  console.log('Initializing nodes...');
});

I see when i call any API to express for fist request i will got

Running at localhost:4000
Initializing nodes...

for second request

Error: listen EADDRINUSE: address already in use :::port

Pls give me any ideas how can resolve this problem