NodeJS/MongoDB - field is being added only once?

I'm learning React with Node (Express/Mongoose) and MongoDB and everything seems to work as expected now but I have this huge issue:

var userSchema = new mongoose.Schema({
  login: { type: String, required: true, unique: true },
  password: { type: String, required: true }
})

app.post('/registerUser', function (req, res) {
  res.send(req.body)

  db.on('error', console.error.bind(console, 'connection error:'))

  // save user using model
  var user = new UserModel(req.body)
  user.save()
}

Every time I POST login and password it gets saved only once and then it's over.

So for example if I send POST three times:

login=admin&password=admin

login=admin2&password=admin2

login=admin3&password=admin3

db.users.count() shows 1 and only the very first POST (admin:admin) is in the database. The rest is being ignored. I'm running console.logs to see what's wrong but every POST gets different _id, but somehow then do not save after the first one. I was thinking maybe there is something wrong with save and maybe I should use something like update or create but every tutorial I've seen and the documentation says to use save.

When I remove the field and the collection is empty and then send POST it saves immediately and then ignores every other POST (second, third, etc.).

What am I doing wrong? I want to have more than 1 user in my collection :(

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