Avoid adding duplicate hint candidates if they are first hops

If we have a direct channel to a node generating an invoice with route
hints, we'd previously happily add multiple candidates that all refer to
the same channel. To keep our candidate set small and unify our tracking
where possible, we now check if its `short_channel_id` is an
`outbound_scid_alias` of any of our first hops and refrain from adding
another candidate if it's the case.

lightning/src/routing/router.rs

@@ -2126,6 +2126,15 @@ where L::Target: Logger {
 				for (idx, (hop, prev_hop_id)) in hop_iter.zip(prev_hop_iter).enumerate() {
 					let source = NodeId::from_pubkey(&hop.src_node_id);
 					let target = NodeId::from_pubkey(&prev_hop_id);
+
+					if let Some(first_channels) = first_hop_targets.get(&target) {
+						if first_channels.iter().any(|d| d.outbound_scid_alias == Some(hop.short_channel_id)) {
+							log_trace!(logger, "Ignoring route hint with SCID {} (and any previous) due to it being a direct channel of ours.",
+								hop.short_channel_id);
+							break;
+						}
+					}
+
 					let candidate = network_channels
 						.get(&hop.short_channel_id)
 						.and_then(|channel| channel.as_directed_to(&target))
@@ -2169,12 +2178,12 @@ where L::Target: Logger {
 						.saturating_add(1);
 
 					// Searching for a direct channel between last checked hop and first_hop_targets
-					if let Some(first_channels) = first_hop_targets.get_mut(&NodeId::from_pubkey(&prev_hop_id)) {
+					if let Some(first_channels) = first_hop_targets.get_mut(&target) {
 						sort_first_hop_channels(first_channels, &used_liquidities,
 							recommended_value_msat, our_node_pubkey);
 						for details in first_channels {
 							let first_hop_candidate = CandidateRouteHop::FirstHop { details };
-							add_entry!(first_hop_candidate, our_node_id, NodeId::from_pubkey(&prev_hop_id),
+							add_entry!(first_hop_candidate, our_node_id, target,
 								aggregate_next_hops_fee_msat, aggregate_path_contribution_msat,
 								aggregate_next_hops_path_htlc_minimum_msat, aggregate_next_hops_path_penalty_msat,
 								aggregate_next_hops_cltv_delta, aggregate_next_hops_path_length);