+ *
+ * While the behavior of [`InvoicePayer`] provides idempotency of duplicate `send_*payment` calls
+ * with the same [`PaymentHash`], it is up to the `Payer` to provide idempotency across restarts.
+ *
+ * [`ChannelManager`] provides idempotency for duplicate payments with the same [`PaymentId`].
+ *
+ * In order to trivially ensure idempotency for payments, the default `Payer` implementation
+ * reuses the [`PaymentHash`] bytes as the [`PaymentId`]. Custom implementations wishing to
+ * provide payment idempotency with a different idempotency key (i.e. [`PaymentId`]) should map
+ * the [`Invoice`] or spontaneous payment target pubkey to their own idempotency key.
+ *
+ * [`ChannelManager`]: lightning::ln::channelmanager::ChannelManager