Why functions-framework are triggered twice when only once visited by a browser?

Question

I installed functions-framework and run a server locally by following this document.

https://cloud.google.com/functions/docs/functions-framework

package.json

"scripts": {
    "start": "functions-framework --target=helloWorld"
  }

index.js

exports.helloWorld = (req, res) => {
  res.send('Hello, World');
};

-

npm start
...
Serving function...
Function: helloWorld
URL: http://localhost:8080/
curl localhost:8080
# Output: Hello, World

It works fine, so I changed index.js to use Puppeteer.

index.js

const puppeteer = require('puppeteer');
exports.helloWorld = async (req, res) => {
    const browser = await puppeteer.launch({headless: false, args: ['--no-sandbox']});
    const page = await browser.newPage();
    page.goto('https://www.google.co.jp');
}

When I use curl localhost:8080, the Chronium browser opens just once. But when I enter localhost:8080 in browser's address bar, it opens twice.

Why the Chronium opens twice?

I checked the behavior by Chrome, Firefox and Opera, so it seems not depend on browsers.