How can I get a duplicate bill of my iPhone XS?

If it was from Apple Store you would have received the invoice on your email, provided by you to them while making the purchase.

With online Order Status you can Print an Invoice

Log into Apple

1. Click on the bag icon as shown in the above
2. Click on Sign in
3. Click on Orders
4. Click on the items that you want an invoice for.
5. Click on View Invoice under the Order Number Top right-hand corner. See the pic below...

There you have it.