Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client in nextJS

The code works properly and redirects to the stripe checkout page but after deployment, it doesn't. I'm getting the status 500 when trying to checkout but the cart items and amount get posted in stripe logs with the status unpaid.

Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client at new NodeError (node:internal/errors:387:5) at ServerResponse.setHeader (node:_http_outgoing:603:11) at NodeNextResponse.setHeader (C:\Users\subash\OneDrive\Desktop\ecommerce\ecommerce\node_modules\next\dist\server\base-http\node.js:56:19)

Here's the code

lib/getStripe.js

import {loadStripe} from '@stripe/stripe-js';

let stripePromise;

const getStripe = () => {
    if(!stripePromise){
        stripePromise = loadStripe(process.env.NEXT_PUBLIC_STRIPE_PUBLISHABLE_KEY);
    }
    return stripePromise;
}

export default getStripe;

cart.js

const handleCheckout = async () => {
    const stripe = await getStripe();

    const response = await fetch('/api/stripe', {
      method: "POST",
      headers: {
        "Content-Type": "application/json"
      },
      body: JSON.stringify(cartItems)
    });

    if (response.statusCode === 500) return;
    const data = await response.json();

    toast.loading("Redirecting...");
    stripe.redirectToCheckout({ sessionId: data.id });
  };

pages/api/stripe.js

import Stripe from 'stripe';

const stripe = new Stripe(process.env.NEXT_PUBLIC_STRIPE_SECRET_KEY);

export default async function handler(req, res) {
  if (req.method === 'POST') {
    
    try {
      // Create Checkout Sessions from body params.
      const params = {
        submit_type :'pay',
        mode:'payment',
        payment_method_types:['card'],
        shipping_address_collection: {
            allowed_countries: ['IN'],
        },
        shipping_options: [
            {shipping_rate: '...'}
        ],
        line_items: req.body.map((item)=>{
            const img = item.image[0].asset._ref;
            const newImage = img.replace('image-','https://cdn.sanity.io/..../').replace('-webp','.webp');
            return {
                price_data:{
                    currency:'inr',
                    product_data:{
                        name:item.name,
                        images:[newImage],
                    },
                    unit_amount:item.price*100,
                },
                adjustable_quantity:{
                    enabled:true,
                    minimum:1
                },
                quantity:item.quantity
            }
        }),
        success_url: `${req.headers.origin}/success`,
        cancel_url: `${req.headers.origin}`,
      }
      const session = await stripe.checkout.sessions.create(params);

      res.status(200).json(session);
      res.redirect(303, session.url);
    } catch (err) {
      res.status(err.statusCode || 500).json(err.message);
    }
  } else {
    res.setHeader('Allow', 'POST');
    console.log("error");
    res.status(405).end('Method Not Allowed');
  }
}