diff --git a/crates/no-mistakes/src/edge_index/adjacency.rs b/crates/no-mistakes/src/edge_index/adjacency.rs
index 29ec9ed26..7a7fc2d5a 100644
--- a/crates/no-mistakes/src/edge_index/adjacency.rs
+++ b/crates/no-mistakes/src/edge_index/adjacency.rs
@@ -144,6 +144,24 @@ pub(crate) fn push_ordinal<Node, Kind>(
     }
 }
 
+pub(crate) fn remember_pair<Node, Kind>(
+    known: &mut FxHashMap<Node, FxHashSet<Kind>>,
+    to: &Node,
+    kind: &Kind,
+) -> bool
+where
+    Node: Clone + Eq + Hash,
+    Kind: Clone + Eq + Hash,
+{
+    if let Some(kinds) = known.get_mut(to) {
+        return kinds.insert(kind.clone());
+    }
+    let mut kinds = FxHashSet::default();
+    kinds.insert(kind.clone());
+    known.insert(to.clone(), kinds);
+    true
+}
+
 pub(crate) fn seed_known_targets<Node, Kind>(
     existing: Option<&Adjacency<Node, Kind>>,
 ) -> FxHashMap<Node, FxHashSet<Kind>>
@@ -151,17 +169,17 @@ where
     Node: Clone + Eq + Hash,
     Kind: Clone + Eq + Hash,
 {
-    match existing {
-        Some(adj) => {
-            let mut known: FxHashMap<Node, FxHashSet<Kind>> =
-                fx_map_with_capacity(adj.neighbors.len());
-            for (to, kind) in &adj.neighbors {
-                known.entry(to.clone()).or_default().insert(kind.clone());
-            }
-            known
-        }
-        None => FxHashMap::default(),
+    let Some(adj) = existing else {
+        return FxHashMap::default();
+    };
+    if adj.neighbors.is_empty() {
+        return FxHashMap::default();
+    }
+    let mut known: FxHashMap<Node, FxHashSet<Kind>> = fx_map_with_capacity(adj.neighbors.len());
+    for (to, kind) in &adj.neighbors {
+        remember_pair(&mut known, to, kind);
     }
+    known
 }
 
 #[cfg(test)]
diff --git a/crates/no-mistakes/src/edge_index/adjacency/tests.rs b/crates/no-mistakes/src/edge_index/adjacency/tests.rs
index f2a18e378..05952c096 100644
--- a/crates/no-mistakes/src/edge_index/adjacency/tests.rs
+++ b/crates/no-mistakes/src/edge_index/adjacency/tests.rs
@@ -1,4 +1,6 @@
-use super::{into_adjacency_map, push_neighbor, push_ordinal, seed_known_targets, Adjacency};
+use super::{
+    into_adjacency_map, push_neighbor, push_ordinal, remember_pair, seed_known_targets, Adjacency,
+};
 use crate::fx::{fx_map, FxHashMap};
 
 fn one_adj(key: &str, adj: Adjacency<String, u8>) -> FxHashMap<String, Adjacency<String, u8>> {
@@ -88,14 +90,22 @@ fn push_ordinal_hit_and_miss_do_not_clone_on_hit() {
 fn seed_known_targets_covers_existing_and_missing_sources() {
     for (existing, expected_len) in [
         (None, 0),
+        (
+            Some(Adjacency {
+                neighbors: Vec::new(),
+                ordinals: Vec::new(),
+            }),
+            0,
+        ),
         (
             Some(Adjacency {
                 neighbors: vec![
                     ("b".to_owned(), 1_u8),
+                    ("b".to_owned(), 1),
                     ("b".to_owned(), 2),
                     ("c".to_owned(), 1),
                 ],
-                ordinals: vec![0, 1, 2],
+                ordinals: vec![0, 1, 2, 3],
             }),
             2,
         ),
@@ -111,6 +121,19 @@ fn seed_known_targets_covers_existing_and_missing_sources() {
     }
 }
 
+#[test]
+fn remember_pair_inserts_once_per_target_kind() {
+    let mut known = fx_map();
+    assert!(remember_pair(&mut known, &"b".to_owned(), &1_u8));
+    assert!(remember_pair(&mut known, &"b".to_owned(), &2));
+    assert!(!remember_pair(&mut known, &"b".to_owned(), &1));
+    assert!(remember_pair(&mut known, &"c".to_owned(), &1));
+    assert_eq!(known.len(), 2);
+    assert!(known
+        .get("b")
+        .is_some_and(|kinds| kinds.contains(&1) && kinds.contains(&2)));
+}
+
 #[test]
 fn into_adjacency_map_preserves_keys_and_presizes_ordinals() {
     let mut input = fx_map();
diff --git a/crates/no-mistakes/src/edge_index/build/extend.rs b/crates/no-mistakes/src/edge_index/build/extend.rs
index 77abddf99..f7471fb5d 100644
--- a/crates/no-mistakes/src/edge_index/build/extend.rs
+++ b/crates/no-mistakes/src/edge_index/build/extend.rs
@@ -1,4 +1,4 @@
-use super::super::adjacency::{push_neighbor, seed_known_targets};
+use super::super::adjacency::{push_neighbor, remember_pair, seed_known_targets, Adjacency};
 use super::super::{CanonicalEdge, EdgeIndex};
 use crate::fx::{fx_map_with_capacity, FxHashMap, FxHashSet};
 use std::hash::Hash;
@@ -21,48 +21,49 @@ where
         let (lower, upper) = edges.size_hint();
         let mut known_by_source = fx_map_with_capacity(upper.unwrap_or(lower));
         for edge in edges {
-            if !known_by_source.contains_key(&edge.from) {
-                known_by_source.insert(
-                    edge.from.clone(),
-                    seed_known_targets(self.forward.get(&edge.from)),
-                );
-            }
-            if pair_is_known(&known_by_source[&edge.from], &edge.to, &edge.kind) {
+            if !record_unknown_pair(
+                &mut known_by_source,
+                &self.forward,
+                &edge.from,
+                &edge.to,
+                &edge.kind,
+            ) {
                 continue;
             }
-            let known = known_by_source
-                .get_mut(&edge.from)
-                .expect("source known-set is inserted before accept");
-            known
-                .entry(edge.to.clone())
-                .or_default()
-                .insert(edge.kind.clone());
             let ordinal = self.edges.len();
+            let CanonicalEdge { from, to, kind } = edge;
             push_neighbor(
                 &mut self.forward,
-                &edge.from,
-                (edge.to.clone(), edge.kind.clone()),
+                &from,
+                (to.clone(), kind.clone()),
                 ordinal,
             );
             push_neighbor(
                 &mut self.reverse,
-                &edge.to,
-                (edge.from.clone(), edge.kind.clone()),
+                &to,
+                (from.clone(), kind.clone()),
                 ordinal,
             );
-            self.edges.push(edge);
+            self.edges.push(CanonicalEdge { from, to, kind });
         }
     }
 }
 
-fn pair_is_known<Node, Kind>(
-    known: &FxHashMap<Node, FxHashSet<Kind>>,
+fn record_unknown_pair<Node, Kind>(
+    known_by_source: &mut FxHashMap<Node, FxHashMap<Node, FxHashSet<Kind>>>,
+    forward: &FxHashMap<Node, Adjacency<Node, Kind>>,
+    from: &Node,
     to: &Node,
     kind: &Kind,
 ) -> bool
 where
-    Node: Eq + Hash,
-    Kind: Eq + Hash,
+    Node: Clone + Eq + Hash,
+    Kind: Clone + Eq + Hash,
 {
-    known.get(to).is_some_and(|kinds| kinds.contains(kind))
+    if let Some(known) = known_by_source.get_mut(from) {
+        return remember_pair(known, to, kind);
+    }
+    let seeded = seed_known_targets(forward.get(from));
+    let known = known_by_source.entry(from.clone()).or_insert(seeded);
+    remember_pair(known, to, kind)
 }
diff --git a/crates/no-mistakes/src/edge_index/tests_extend.rs b/crates/no-mistakes/src/edge_index/tests_extend.rs
index 46aec4a1d..e550580f4 100644
--- a/crates/no-mistakes/src/edge_index/tests_extend.rs
+++ b/crates/no-mistakes/src/edge_index/tests_extend.rs
@@ -85,16 +85,35 @@ fn extend_edges_preserving_ordinals_matches_rebuild_cases() {
 fn extend_edges_from_seeded_empty_source_hits_existing_adjacency() {
     let mut index =
         EdgeIndex::from_edges_and_nodes(std::iter::empty(), ["a".to_owned(), "orphan".to_owned()]);
-    index.extend_edges_preserving_ordinals([edge("a", "b", 1), edge("a", "b", 1)]);
-    assert_eq!(index.edges(), &[edge("a", "b", 1)]);
+    index.extend_edges_preserving_ordinals([
+        edge("a", "b", 1),
+        edge("a", "b", 1),
+        edge("a", "b", 2),
+        edge("a", "c", 3),
+        edge("z", "b", 4),
+        edge("z", "b", 4),
+    ]);
+    assert_eq!(
+        index.edges(),
+        &[
+            edge("a", "b", 1),
+            edge("a", "b", 2),
+            edge("a", "c", 3),
+            edge("z", "b", 4)
+        ]
+    );
     assert_eq!(
         index.forward().get("a").map(|adj| adj.ordinals.as_slice()),
-        Some(&[0][..])
+        Some(&[0, 1, 2][..])
     );
     assert!(index
         .forward()
         .get("orphan")
         .is_some_and(|adj| adj.neighbors.is_empty()));
+    assert_eq!(
+        index.reverse().get("b").map(|adj| adj.ordinals.as_slice()),
+        Some(&[0, 1, 3][..])
+    );
 }
 
 #[test]