diff --git a/src/app/shared/astro/host-star-matching.spec.ts b/src/app/shared/astro/host-star-matching.spec.ts
index fef3a2d..c08c96c 100644
--- a/src/app/shared/astro/host-star-matching.spec.ts
+++ b/src/app/shared/astro/host-star-matching.spec.ts
@@ -47,14 +47,18 @@ describe('resolveHostStarId', () => {
   describe('matching on the sky', () => {
     // GJ 887's archive row: position at Gaia's epoch, carried by 6.9″/yr of proper motion —
     // 110″ from where the catalogue has the star at J2000. The matcher must carry the query
-    // back those sixteen years itself.
-    it('matches a host published at the Gaia epoch to its star at J2000', () => {
+    // back those sixteen years itself, and judge each star on the better of the two epochs: the
+    // decoy standing halfway along the star's own track is nearer than Lacaille 9352 at the
+    // published point *and* nearer at the worse of the two epochs, so it wins unless the
+    // carried-back position is tried and the best epoch — not the worst — decides.
+    it('matches a host published at the Gaia epoch to its star at J2000, past a decoy on its track', () => {
       const lacaille9352 = star(70, 'Lacaille 9352', 346.46683, -35.85306, 3.29);
       const archive = propagateProperMotion(346.46683, -35.85306, 6768.2, 1327.52, 16);
+      const decoy = star(71, 'Decoy', (346.46683 + archive.raDeg) / 2, (-35.85306 + archive.decDeg) / 2, 3.29);
 
       const id = resolveHostStarId(
         { hostname: 'GJ 887', raDeg: archive.raDeg, decDeg: archive.decDeg, distancePc: 3.28679, pmRaMasPerYear: 6768.2, pmDecMasPerYear: 1327.52 },
-        [lacaille9352]
+        [decoy, lacaille9352]
       );
 
       expect(id).toBe(70);
@@ -121,6 +125,45 @@ describe('resolveHostStarId', () => {
       expect(at50).toBe(111);
     });
 
+    // A star whose distance disqualifies it is not merely rejected — it must not become the
+    // best-so-far either, or an unmerged twin with a bad parallax, sitting nearer on the sky
+    // than the true host, silently unhosts the planet by outranking a star that is never
+    // allowed to win.
+    it('does not let a star its distance disqualifies shadow the true host behind it', () => {
+      const badParallaxTwin = star(120, 'Gaia DR3 twin', 40, 12 + 1 / 3600, 480);
+      const host = star(121, 'True host', 40, 12 + 3 / 3600, 100);
+
+      const id = resolveHostStarId({ hostname: 'Unmatched', raDeg: 40, decDeg: 12, distancePc: 100 }, [badParallaxTwin, host]);
+
+      expect(id).toBe(121);
+    });
+
+    // A proper motion that is not a number must not poison the comparison: NaN loses every
+    // `<` it appears in, so an unguarded one lets each star past the direction test and hands
+    // the planet to whichever happens to be last in the catalogue.
+    it('treats an unusable proper motion as no motion rather than matching by array order', () => {
+      const pointedAt = star(130, 'Pointed at', 10, 10, 5);
+      const acrossTheSky = star(131, 'Across the sky', 190, -10, 5);
+
+      const id = resolveHostStarId(
+        { hostname: 'Unmatched', raDeg: 10, decDeg: 10, distancePc: 5, pmRaMasPerYear: NaN, pmDecMasPerYear: 0 },
+        [pointedAt, acrossTheSky]
+      );
+
+      expect(id).toBe(130);
+    });
+
+    // Normalizing strips the dot, so `Gl 55.2` and `Gl 552` — two stars 135° apart — answer to
+    // one key. A name that names both names neither: the sky has to settle it.
+    it('sends a name two stars answer to back to the sky', () => {
+      const gl552 = star(140, 'Gl 552', 217.0, 15.0, 14.2);
+      const gl55dot2 = star(141, 'Gl 55.2', 30.0, -20.0, 23.9);
+
+      const id = resolveHostStarId({ hostname: 'Gl 552', raDeg: 217.0, decDeg: 15.0, distancePc: 14.2 }, [gl552, gl55dot2]);
+
+      expect(id).toBe(140);
+    });
+
     it('reuses a prebuilt name index when given one', () => {
       const nameIndex = buildStarNameIndex(FIXTURE_STARS);
 
diff --git a/src/app/shared/astro/host-star-matching.ts b/src/app/shared/astro/host-star-matching.ts
index c2c5041..01e3e88 100644
--- a/src/app/shared/astro/host-star-matching.ts
+++ b/src/app/shared/astro/host-star-matching.ts
@@ -17,9 +17,29 @@ export interface HostStarQuery {
   pmDecMasPerYear?: number;
 }
 
-/** Builds a lookup of normalized star name -> star, for fast repeated name matching. */
+/**
+ * Builds a lookup of normalized star name -> star, for fast repeated name matching.
+ *
+ * A name two stars answer to names neither: normalizing strips the dot, so `Gl 55.2` and
+ * `Gl 552` — 135° apart, and 64 such groups exist in the catalogue — collide on `gl552`, and a
+ * map would silently keep whichever came last. Ambiguous keys are dropped instead, which sends
+ * the query to the sky, where direction settles it.
+ */
 export function buildStarNameIndex(stars: readonly StarRecord[]): Map<string, StarRecord> {
-  return new Map(stars.map((star) => [normalizeStarName(star.name), star]));
+  const index = new Map<string, StarRecord>();
+  const ambiguous = new Set<string>();
+  for (const star of stars) {
+    const key = normalizeStarName(star.name);
+    if (index.has(key)) {
+      ambiguous.add(key);
+    } else {
+      index.set(key, star);
+    }
+  }
+  for (const key of ambiguous) {
+    index.delete(key);
+  }
+  return index;
 }
 
 /**
@@ -55,6 +75,10 @@ export const HOST_TRANSVERSE_TOLERANCE_PC = 0.01;
 const CATALOGUE_EPOCH = 2000.0;
 const ARCHIVE_LATEST_EPOCH = 2016.0;
 
+function knownMotion(masPerYear: number | undefined): number {
+  return Number.isFinite(masPerYear) ? (masPerYear as number) : 0;
+}
+
 /**
  * Cross-references an exoplanet host star to the star catalogue: first by (normalized) name,
  * then on the sky — the nearest star within {@link HOST_TRANSVERSE_TOLERANCE_PC} whose distance
@@ -96,8 +120,11 @@ export function resolveHostStarId(
   const carriedBack = propagateProperMotion(
     query.raDeg,
     query.decDeg,
-    query.pmRaMasPerYear ?? 0,
-    query.pmDecMasPerYear ?? 0,
+    // A proper motion that is not a number must read as "stands still", not poison the
+    // comparison: one NaN makes every star's cosine NaN, and `NaN < min` is false, so every
+    // star would pass the direction test and the last one in array order would win.
+    knownMotion(query.pmRaMasPerYear),
+    knownMotion(query.pmDecMasPerYear),
     CATALOGUE_EPOCH - ARCHIVE_LATEST_EPOCH
   );
   const carried = raDegDecDistanceToXyz(carriedBack.raDeg, carriedBack.decDeg, 1);
diff --git a/tools/etl/fetchExoplanets.ts b/tools/etl/fetchExoplanets.ts
index 4c638fb..767c935 100644
--- a/tools/etl/fetchExoplanets.ts
+++ b/tools/etl/fetchExoplanets.ts
@@ -35,10 +35,11 @@ const TAP_COLUMNS = [
 const TAP_QUERY = `select+${TAP_COLUMNS}+from+ps+where+default_flag=1+order+by+pl_name&format=csv`;
 const TAP_URL = `${TAP_BASE_URL}?query=${TAP_QUERY}`;
 
-// The cache is keyed by the query it answers: a response cached before a column was added
-// would otherwise keep serving rows without it, and a missing proper-motion cell reads as
-// "does not move" — silently wrong, not visibly broken.
-const CACHE_FILE = `exoplanet-archive-ps-${createHash('sha1').update(TAP_QUERY).digest('hex').slice(0, 8)}.csv`;
+// The cache is keyed by the request it answers — endpoint included, since the cache records
+// only that some response arrived: one cached before a column was added would otherwise keep
+// serving rows without it, and a missing proper-motion cell reads as "does not move",
+// silently wrong rather than visibly broken.
+const CACHE_FILE = `exoplanet-archive-ps-${createHash('sha1').update(TAP_URL).digest('hex').slice(0, 8)}.csv`;
 
 /**
  * Downloads confirmed exoplanets from the NASA Exoplanet Archive (`Planetary Systems` TAP
diff --git a/tools/etl/sources/gaia.ts b/tools/etl/sources/gaia.ts
index a4fb700..ebcc255 100644
--- a/tools/etl/sources/gaia.ts
+++ b/tools/etl/sources/gaia.ts
@@ -87,9 +87,10 @@ export async function fetchGaiaStars(): Promise<StarRecord[]> {
   const url = `${GAIA_TAP_URL}?REQUEST=doQuery&LANG=ADQL&FORMAT=csv&QUERY=${encodeURIComponent(query)}`;
   console.log(`Fetching Gaia DR3 (within ${DISTANCE_CUTOFF_PC} pc, G < ${MAGNITUDE_LIMIT}, at most ${ROW_LIMIT} rows)...`);
 
-  // Keyed by the query itself, so a response cached for other columns or another order can
-  // never be mistaken for this one.
-  const csv = await fetchTextCached(url, `gaia-dr3-${createHash('sha1').update(query).digest('hex').slice(0, 8)}.csv`);
+  // Keyed by the whole request, so a response cached for other columns, another order, or
+  // another endpoint can never be mistaken for this one — the cache records only that some
+  // response arrived, not what it answered.
+  const csv = await fetchTextCached(url, `gaia-dr3-${createHash('sha1').update(url).digest('hex').slice(0, 8)}.csv`);
   const rows = parseCsvObjects(csv);
   const stars: StarRecord[] = [];