Add test for duplicate keysend payment

The logic has been changed around duplicate keysend payments such that
it's no longer explicitly clear that we reject duplicate keysend
payments now that we handle receiving multi-part keysends. This test
catches that. Note that this also tests that we reject MPP keysends when
our config states we should, and that we reject MPP keysends without
payemnt secrets when our config states we support MPP keysends.

lightning/src/ln/channelmanager.rs

@@ -8796,13 +8796,26 @@ mod tests {
 
 	#[test]
 	fn test_keysend_dup_payment_hash() {
+		do_test_keysend_dup_payment_hash(false);
+		do_test_keysend_dup_payment_hash(true);
+	}
+
+	fn do_test_keysend_dup_payment_hash(accept_mpp_keysend: bool) {
 		// (1): Test that a keysend payment with a duplicate payment hash to an existing pending
 		//      outbound regular payment fails as expected.
 		// (2): Test that a regular payment with a duplicate payment hash to an existing keysend payment
 		//      fails as expected.
+		// (3): Test that a keysend payment with a duplicate payment hash to an existing keysend
+		//      payment fails as expected. When `accept_mpp_keysend` is false, this tests that we
+		//      reject MPP keysend payments, since in this case where the payment has no payment
+		//      secret, a keysend payment with a duplicate hash is basically an MPP keysend. If
+		//      `accept_mpp_keysend` is true, this tests that we only accept MPP keysends with
+		//      payment secrets and reject otherwise.
 		let chanmon_cfgs = create_chanmon_cfgs(2);
 		let node_cfgs = create_node_cfgs(2, &chanmon_cfgs);
-		let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, None]);
+		let mut mpp_keysend_cfg = test_default_channel_config();
+		mpp_keysend_cfg.accept_mpp_keysend = accept_mpp_keysend;
+		let node_chanmgrs = create_node_chanmgrs(2, &node_cfgs, &[None, Some(mpp_keysend_cfg)]);
 		let nodes = create_network(2, &node_cfgs, &node_chanmgrs);
 		create_announced_chan_between_nodes(&nodes, 0, 1);
 		let scorer = test_utils::TestScorer::new();
@@ -8891,6 +8904,53 @@ mod tests {
 
 		// Finally, succeed the keysend payment.
 		claim_payment(&nodes[0], &expected_route, payment_preimage);
+
+		// To start (3), send a keysend payment but don't claim it.
+		let payment_id_1 = PaymentId([44; 32]);
+		let payment_hash = nodes[0].node.send_spontaneous_payment(&route, Some(payment_preimage),
+			RecipientOnionFields::spontaneous_empty(), payment_id_1).unwrap();
+		check_added_monitors!(nodes[0], 1);
+		let mut events = nodes[0].node.get_and_clear_pending_msg_events();
+		assert_eq!(events.len(), 1);
+		let event = events.pop().unwrap();
+		let path = vec![&nodes[1]];
+		pass_along_path(&nodes[0], &path, 100_000, payment_hash, None, event, true, Some(payment_preimage));
+
+		// Next, attempt a keysend payment and make sure it fails.
+		let route_params = RouteParameters {
+			payment_params: PaymentParameters::for_keysend(expected_route.last().unwrap().node.get_our_node_id(), TEST_FINAL_CLTV, false),
+			final_value_msat: 100_000,
+		};
+		let route = find_route(
+			&nodes[0].node.get_our_node_id(), &route_params, &nodes[0].network_graph,
+			None, nodes[0].logger, &scorer, &(), &random_seed_bytes
+		).unwrap();
+		let payment_id_2 = PaymentId([45; 32]);
+		nodes[0].node.send_spontaneous_payment(&route, Some(payment_preimage),
+			RecipientOnionFields::spontaneous_empty(), payment_id_2).unwrap();
+		check_added_monitors!(nodes[0], 1);
+		let mut events = nodes[0].node.get_and_clear_pending_msg_events();
+		assert_eq!(events.len(), 1);
+		let ev = events.drain(..).next().unwrap();
+		let payment_event = SendEvent::from_event(ev);
+		nodes[1].node.handle_update_add_htlc(&nodes[0].node.get_our_node_id(), &payment_event.msgs[0]);
+		check_added_monitors!(nodes[1], 0);
+		commitment_signed_dance!(nodes[1], nodes[0], payment_event.commitment_msg, false);
+		expect_pending_htlcs_forwardable!(nodes[1]);
+		expect_pending_htlcs_forwardable_and_htlc_handling_failed!(nodes[1], vec![HTLCDestination::FailedPayment { payment_hash }]);
+		check_added_monitors!(nodes[1], 1);
+		let updates = get_htlc_update_msgs!(nodes[1], nodes[0].node.get_our_node_id());
+		assert!(updates.update_add_htlcs.is_empty());
+		assert!(updates.update_fulfill_htlcs.is_empty());
+		assert_eq!(updates.update_fail_htlcs.len(), 1);
+		assert!(updates.update_fail_malformed_htlcs.is_empty());
+		assert!(updates.update_fee.is_none());
+		nodes[0].node.handle_update_fail_htlc(&nodes[1].node.get_our_node_id(), &updates.update_fail_htlcs[0]);
+		commitment_signed_dance!(nodes[0], nodes[1], updates.commitment_signed, true, true);
+		expect_payment_failed!(nodes[0], payment_hash, true);
+
+		// Finally, claim the original payment.
+		claim_payment(&nodes[0], &expected_route, payment_preimage);
 	}
 
 	#[test]