Unable to set headers after sending them ... no problem until I add authentication?

Question

I'm having trouble sending requests using custom headers from my application so that I can use them for authentication on my server.

I have a built-in response application that uses firebase validation and a node / express backend server.

When the user writes, I will take their firebase idtoken and save it.

Then I will send this idtoken as a header whenever I make an axiom request to the server.

 const { currentUser } = firebase.auth();

  const url = ${serverUrl}/api/users/${user.uid`;

  const idtoken = await currentUser.getToken(true);

    var config = {
      headers: { idtoken },
    };

const response = await axios(url, config);

, , idtoken, , . , next() . , 401.

app.js:

const app = express();


  app.use(bodyParser.json());

  app.use(firebaseAuthentication);
  router(app);

firebaseAuthentication:

const firebaseAuthentication = async (req, res, next) => {
  console.log(req);
  try {
    const { idtoken } = req.headers;

    await admin.auth().verifyIdToken(idtoken);
    next();
  } catch (e) {
    res.status(401).send({ error: 'Unauthorized request. Must be signed in via firebase' });
  }
};

:

module.exports = app => {

    app.get('/api/users/:firebaseUID', UserController.getUser);
}

, , , , ... 404 Error: Can't set headers after they are sent

, :

  async getUser(req, res, next) {
    const firebaseUID = req.params.firebaseUID;
    try {
      const user = await User.findOne({ firebaseUID });

      res.send(user);
    } catch (e) {
      next(e);
    }
  },

? !