How to ensure that the execution of functions are in order?

I have a HTTP-triggered function which returns an increment number every time the endpoint is called. The code looks like this:

export const reserve = functions.https.onRequest((req, resp) => {
  cors(req, resp, async () => {
    if (req.method.toLowerCase() !== 'post') {
      resp.status(405);
      resp.end();
    } else {
      const path = `counter`;
      const ref = firebase.database().ref(path);
      const oldCount = (await ref.once('value')).val();
      await ref.set(oldCount + 1);
      resp.status(200).send({
        number: oldCount
      });
      resp.end();
    }
  });
});

The problem is, if 2 invocations are very close to each other, is there a chance that the function can return the same number? If so, is there a way to prevent this?