Add a separate PaymentSendFailure for idempotency violation

Add a separate PaymentSendFailure for idempotency violation

When a user attempts to send a payment but it fails due to
idempotency key violation, they need to know that this was the
reason as they need to handle the error programmatically
differently from other errors.

Here we simply add a new `PaymentSendFailure` enum variant for
`DuplicatePayment` to allow for that.