[w3c/browser-payment-api] Specify UA requirements if “id” field of PaymentShippingOption is not unique (#243)

The [spec section defining the `PaymentShippingOption` dictionary](https://w3c.github.io/browser-payment-api/#paymentshippingoption-dictionary) states, for the `id` field:

> This is a string identifier used to reference this `PaymentShippingOption`. It MUST be unique for a given `PaymentRequest`.

However, the spec doesn’t explicitly define what UAs must do if a `PaymentRequest` constructor is called with a `PaymentShippingOption` instance that has a non-unique (that is, duplicate) `id` field.

It seems like this could be fixed by adding a statement of the specific requirement in the [“PaymentRequest constructor must act as follows“](https://w3c.github.io/browser-payment-api/#paymentrequest-constructor) steps.

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