How to ensure that an Idempotency Key is the same in subsequent requests?

I am going over methods of handling failures in distributed systems for interview preparation. I have never implemented an idempotency key.

I have gone over the explanation in this hellointerview article and this ByteMonk video. The explanations lead me to think that it is the client that has to store the idempotency key. However, I don't see how this is done in the Javascript code provided in 3:48.

If the client sends an order again but clicking pay a second time, won't the client generate a new uuid and thus create a new payment.

const paypal = require('@paypal/checkout-server-sdk');

// ... (Set up your PayPal environment and client credentials)

// Create a request object with an idempotency key
const request = new paypal.orders.OrdersCreateRequest();
request.headers['prefer'] = 'return=representation';
request.requestBody({
  intent: 'CAPTURE',
  purchase_units: [{
    amount: {
      currency_code: 'USD',
      value: '100.00'
    }
  }]
});

// Generate a unique idempotency key (e.g., using a UUID library)
const idempotencyKey = uuidv4(); // Replace with your preferred UUID generation

// Add the idempotency key to the request headers
request.headers['PayPal-Request-Id'] = idempotencyKey;

// Call the PayPal API to create the order
let createOrderResponse = await client.execute(request);

console.log('Order created with ID:', createOrderResponse.result.id);