Add documentation about PaymentSent/Failed event duplication bug

Author: TheBlueMatt

src/util/events.rs

@@ -60,6 +60,8 @@ pub enum Event {
 	},
 	/// Indicates an outbound payment we made succeeded (ie it made it all the way to its target
 	/// and we got back the payment preimage for it).
+	/// Note that duplicative PaymentSent Events may be generated - it is your responsibility to
+	/// deduplicate them by payment_preimage (which MUST be unique)!
 	PaymentSent {
 		/// The preimage to the hash given to ChannelManager::send_payment.
 		/// Note that this serves as a payment receipt, if you wish to have such a thing, you must
@@ -68,6 +70,8 @@ pub enum Event {
 	},
 	/// Indicates an outbound payment we made failed. Probably some intermediary node dropped
 	/// something. You may wish to retry with a different route.
+	/// Note that duplicative PaymentFailed Events may be generated - it is your responsibility to
+	/// deduplicate them by payment_hash (which MUST be unique)!
 	PaymentFailed {
 		/// The hash which was given to ChannelManager::send_payment.
 		payment_hash: [u8; 32],