Headers Error while making discord authorization page (javascript)

Im making a website with dashboard for a discord bot. Only when im making the authorization page with the help of JWT to make a token it keeps giving me a headers error (see below). Ill put my code for that specific item under here. Can someone please help me out because I can’t seem to find any source of why said error pops up.

---

Error:

node:_http_outgoing:703
    throw new ERR_HTTP_HEADERS_SENT('set');
          ^

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client
    at ServerResponse.setHeader (node:_http_outgoing:703:11)
    at ServerResponse.header (C:xampphtdocsdiscordapiapinode_modulesexpresslibresponse.js:795:10)
    at ServerResponse.send (C:xampphtdocsdiscordapiapinode_modulesexpresslibresponse.js:175:12)
    at ServerResponse.json (C:xampphtdocsdiscordapiapinode_modulesexpresslibresponse.js:279:15)
    at C:xampphtdocsdiscordapiapisrcroutesauthindex.js:109:32
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  code: 'ERR_HTTP_HEADERS_SENT'
}

Node.js v21.6.0
Failed running '.'

Code:

const express = require('express');
const User = require('../../models/User');
const jwt = require('jsonwebtoken');

const router = express.Router();

// GET: designy.xyz/auth/signin
// GET: designy.xyz/auth/callback

const DASHBOARD_URL = 'http://localhost:5173';

router.get('/signin', (req, res) => {
    res.redirect(
        `https://discord.com/oauth2/authorize?client_id=${process.env.DISCORD_CLIENT_ID}&response_type=code&redirect_uri=http%3A%2F%2Flocalhost%3A3001%2Fauth%2Fcallback&scope=guilds+identify`
    );
});

router.get('/callback', async (req, res) => {
    const DISCORD_ENDPOINT = 'https://discord.com/api/v10';
    const CLIENT_ID = process.env.DISCORD_CLIENT_ID;
    const CLIENT_SECRET = process.env.DISCORD_CLIENT_SECRET;
    const REDIRECT_URI = process.env.DISCORD_REDIRECT_URI;
    const DASHBOARD_URL = process.env.DASHBOARD_URL;

    const { code } = req.query;

    if (!code) {
        return res.status(400).json({
            error: 'A "code" query parameter must be present in the URL.',
        });
    }

    try {
        const oauthRes = await fetch(`${DISCORD_ENDPOINT}/oauth2/token`, {
            method: 'POST',
            body: new URLSearchParams({
                client_id: CLIENT_ID,
                client_secret: CLIENT_SECRET,
                grant_type: 'authorization_code',
                redirect_uri: REDIRECT_URI,
                code,
            }).toString(),
            headers: {
                'Content-Type': 'application/x-www-form-urlencoded',
            },
        });

        if (!oauthRes.ok) {
            return res.status(500).json({
                error: 'Failed to obtain OAuth token.',
            });
        }

        const oauthResJson = await oauthRes.json();

        const userRes = await fetch(`${DISCORD_ENDPOINT}/users/@me`, {
            method: 'GET',
            headers: {
                Authorization: `Bearer ${oauthResJson.access_token}`,
            },
        });

        if (!userRes.ok) {
            return res.status(500).json({
                error: 'Failed to fetch user data.',
            });
        }

        const userResJson = await userRes.json();

        let user = await User.findOne({ id: userResJson.id });

        if (!user) {
            user = new User({
                id: userResJson.id,
                username: userResJson.username,
                avatarHash: userResJson.avatar,
                accessToken: oauthResJson.access_token,
                refreshToken: oauthResJson.refresh_token,
            });
        } else {
            user.username = userResJson.username;
            user.avatarHash = userResJson.avatar;
            user.accessToken = oauthResJson.access_token;
            user.refreshToken = oauthResJson.refresh_token;
        }

        await user.save();

        const token = jwt.sign(
            {
                id: userResJson.id,
                username: userResJson.username,
                avatarHash: userResJson.avatar,
            },
            process.env.JWT_SECRET,
            { expiresIn: '7d' }
        );

        res.cookie('access_token', token, {
            httpOnly: true,
            secure: process.env.NODE_ENV === 'production',
            maxAge: 6.048e8,
        });

        return res.redirect(DASHBOARD_URL);
    } catch (error) {
        console.error(error);
        return res.status(500).json({
            error: 'Internal Server Error.',
        });
    }
});

router.get('/signout', (req, res) => {
    res.clearCookie('token').sendStatus(200);
});

module.exports = router;

Help would be much appreciated!

Ive tried to remove the .send .json. status to see if that would fix the issue.

Ive also tried to remove the res.redirect + token to see if that would’ve helped

Both of these things did not work, also tried some other things but i can’t remember them anymore.