diff --git a/AGENTS.md b/AGENTS.md
index dd072601..bd6fff8d 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -315,7 +315,9 @@ node scripts/geocode-places.mjs --check   # exit 1 if anything is unplaced
 ```
 
 Coordinates live in `config/places.json` so the site and CI never call a
-geocoder. The script skips anything not in the format above, and checks the
+geocoder. A name that is the same campus as another — "University of Chicago" and
+"The University of Chicago" — is written `{ "sameAs": "<other name>" }`, not a
+second copy of the coordinates; `tests/places.test.js` fails on a repeat. The script skips anything not in the format above, and checks the
 `matched` string it wrote — Nominatim answers "Durham, UK" with the county, not
 the city. Correct a wrong pin by hand; the script never overwrites one.
 
diff --git a/config/places.json b/config/places.json
index 6fe572b2..75c6d38a 100644
--- a/config/places.json
+++ b/config/places.json
@@ -1,5 +1,5 @@
 {
-  "$comment": "Coordinates for every place the site pins — collaborator institutions and presentation locations alike — resolved once with scripts/geocode-places.mjs and committed so the site and CI never call a geocoder. `matched` is what the geocoder thought it found — check it before trusting a pin. Safe to correct by hand: the script only fills in what is missing.",
+  "$comment": "Coordinates for every place the site pins — collaborator institutions and presentation locations alike — resolved once with scripts/geocode-places.mjs and committed so the site and CI never call a geocoder. `matched` is what the geocoder thought it found — check it before trusting a pin. Safe to correct by hand: the script only fills in what is missing. A name that is the same campus as another is `{ \"sameAs\": \"<other name>\" }` rather than a second copy of the coordinates; read places through src/lib/places.js. Keep one entry per campus: tests/places.test.js fails on two entries with the same coordinates.",
   "source": "https://nominatim.openstreetmap.org (ODbL)",
   "places": {
     "A Coruña, Spain": {
@@ -38,9 +38,7 @@
       "matched": "California Institute of Technology, Moore Walk, Pasadena Council District 7, Pasadena, Los Angeles County, California, 91125, United States"
     },
     "Caltech": {
-      "lat": 34.137,
-      "lon": -118.1253,
-      "matched": "California Institute of Technology, Broad Walk, Pasadena Council District 7, Pasadena, Los Angeles County, California, 91106, United States"
+      "sameAs": "California Institute of Technology"
     },
     "Caltech/JPL": {
       "lat": 34.2015,
@@ -58,9 +56,7 @@
       "matched": "hand-placed: MaRS Centre, 661 University Ave, Toronto"
     },
     "Canadian Institute for Theoretical Astrophysics": {
-      "lat": 43.66,
-      "lon": -79.3971,
-      "matched": "hand-placed: 60 St George St, University of Toronto"
+      "sameAs": "University of Toronto"
     },
     "Cardiff University": {
       "lat": 51.5067,
@@ -73,9 +69,7 @@
       "matched": "hand-placed: 1530 P St NW, Washington DC — Nominatim has no entry"
     },
     "Carnegie Observatories": {
-      "lat": 34.1563,
-      "lon": -118.1334,
-      "matched": "The Carnegie Observatories, Earlham Street, Bungalow Heaven, Pasadena Council District 5, Pasadena, Los Angeles County, California, 91188, United States"
+      "sameAs": "Observatories of the Carnegie Institution of Washington"
     },
     "Case Western Reserve University": {
       "lat": 41.5014,
@@ -88,9 +82,7 @@
       "matched": "60 Garden Street, Cambridge, MA, USA (placed by hand)"
     },
     "Center for Computational Astrophysics": {
-      "lat": 40.7397,
-      "lon": -73.9903,
-      "matched": "hand-placed: part of the Flatiron Institute, 162 Fifth Ave, New York"
+      "sameAs": "Flatiron Institute"
     },
     "Centre de Recherche Astrophysique de Lyon": {
       "lat": 45.6936,
@@ -103,9 +95,7 @@
       "matched": "Centro de Estudios de Física del Cosmos de Aragón, VF-TE-02, Arcos de las Salinas, Gúdar-Javalambre, Teruel, Aragón, España"
     },
     "CESR": {
-      "lat": 43.5626,
-      "lon": 1.4759,
-      "matched": "Toulouse, France (placed by hand) — same place as Institut de Recherche en Astrophysique et Planétologie, its successor"
+      "sameAs": "Institut de Recherche en Astrophysique et Planétologie"
     },
     "Chengdu Experimental Foreign Languages School": {
       "lat": 30.6587,
@@ -213,9 +203,7 @@
       "matched": "Jesi, Ancona, Marche, 60035, Italia"
     },
     "Københavns Universitet": {
-      "lat": 55.6802,
-      "lon": 12.5723,
-      "matched": "Københavns Universitet, Copenhagen (geocoded) — same place as University of Copenhagen"
+      "sameAs": "University of Copenhagen"
     },
     "Laboratoire d'Astrophysique de Marseille": {
       "lat": 43.3436,
@@ -398,9 +386,7 @@
       "matched": "City of Rochester, Monroe County, New York, United States"
     },
     "Ruprecht Karls Universität Heidelberg": {
-      "lat": 49.4186,
-      "lon": 8.6692,
-      "matched": "Universität Heidelberg, Handschuhsheimer Landstraße, Handschuhsheim, Heidelberg, Baden-Württemberg, 69121, Deutschland"
+      "sameAs": "Ruprecht-Karls-Universität Heidelberg"
     },
     "Ruprecht-Karls-Universität Heidelberg": {
       "lat": 49.4186,
@@ -408,9 +394,7 @@
       "matched": "Universität Heidelberg, Handschuhsheimer Landstraße, Handschuhsheim, Heidelberg, Baden-Württemberg, 69121, Deutschland"
     },
     "Rutgers The State University of New Jersey": {
-      "lat": 40.4969,
-      "lon": -74.449,
-      "matched": "Rutgers University, New Brunswick, NJ (geocoded)"
+      "sameAs": "Rutgers University"
     },
     "Rutgers University": {
       "lat": 40.5204,
@@ -433,14 +417,10 @@
       "matched": "Seattle, King County, Washington, United States"
     },
     "Simons Foundation": {
-      "lat": 40.7392,
-      "lon": -73.9909,
-      "matched": "hand-placed: 160 Fifth Ave, New York — Nominatim has no entry"
+      "sameAs": "Flatiron Institute"
     },
     "Smithsonian Astrophysical Observatory": {
-      "lat": 42.3814,
-      "lon": -71.1281,
-      "matched": "60 Garden Street, Cambridge, MA, USA (placed by hand) — same place as Center for Astrophysics Harvard & Smithsonian"
+      "sameAs": "Center for Astrophysics Harvard & Smithsonian"
     },
     "Smithsonian Institution": {
       "lat": 38.8888,
@@ -528,9 +508,7 @@
       "matched": "Orto Botanico dell'Università degli Studi di Cagliari, Stampaxi/Stampace, Casteddu/Cagliari, Sardigna/Sardegna, Italia"
     },
     "Università di Bologna": {
-      "lat": 44.4983,
-      "lon": 11.3545,
-      "matched": "Università di Bologna, Via dei Bibiena, Ghetto ebraico, Irnerio, Santo Stefano, Bologna, Emilia-Romagna, 40126, Italia"
+      "sameAs": "University of Bologna"
     },
     "Universität Bern": {
       "lat": 46.951,
@@ -593,9 +571,7 @@
       "matched": "University of Cambridge, 82-88, Hills Road, Petersfield, Cambridge, Cambridgeshire, Cambridgeshire and Peterborough, England, CB2 1NF, United Kingdom"
     },
     "University of Chicago": {
-      "lat": 41.7881,
-      "lon": -87.6048,
-      "matched": "University of Chicago, 5841, South Maryland Avenue, Hyde Park, Chicago, Hyde Park Township, Cook County, Illinois, 60637, United States"
+      "sameAs": "The University of Chicago"
     },
     "University of Colorado": {
       "lat": 40.0076,
@@ -708,9 +684,7 @@
       "matched": "Université Paris Cité – Campus Saint-Germain-des-Prés, 45, Rue des Saints-Pères, Quartier Saint-Thomas-d'Aquin, Paris 7e Arrondissement, Paris, Île-de-France, France métropolitaine, 75006, France"
     },
     "Universtity of Wisconsin": {
-      "lat": 43.0766,
-      "lon": -89.4125,
-      "matched": "hand-placed: University of Wisconsin-Madison. The name is misspelt on the ORCID record it came from, and is kept as written so the collector still matches it."
+      "sameAs": "University of Wisconsin Madison"
     },
     "Valencia, Spain": {
       "lat": 39.4697,
diff --git a/scripts/geocode-places.mjs b/scripts/geocode-places.mjs
index 1f3c1d41..753d213a 100644
--- a/scripts/geocode-places.mjs
+++ b/scripts/geocode-places.mjs
@@ -3,7 +3,8 @@
 //
 // Both live in one file because they are the same kind of fact — somewhere on
 // Earth the site draws a dot for — and one committed answer per place means the
-// two maps cannot disagree about where Cambridge is.
+// two maps cannot disagree about where Cambridge is. A name that is the same
+// campus as another is `{ sameAs }` it, so each coordinate is written once.
 //
 // Issue #22: geocoding at build time would make CI depend on a third-party
 // API, which this repo avoids everywhere else. So this is run by hand and the
@@ -94,7 +95,10 @@ fs.writeFileSync(OUT, `${JSON.stringify({
     + 'presentation locations alike — resolved once with scripts/geocode-places.mjs and '
     + 'committed so the site and CI never call a geocoder. `matched` is what the geocoder '
     + 'thought it found — check it before trusting a pin. Safe to correct by hand: the '
-    + 'script only fills in what is missing.',
+    + 'script only fills in what is missing. '
+    + 'A name that is the same campus as another is `{ "sameAs": "<other name>" }` rather than a '
+    + 'second copy of the coordinates; read places through src/lib/places.js. Keep one entry per '
+    + 'campus: tests/places.test.js fails on two entries with the same coordinates.',
   source: 'https://nominatim.openstreetmap.org (ODbL)',
   places: Object.fromEntries(Object.entries(places).sort(([a], [b]) => a.localeCompare(b))),
 }, null, 2)}\n`);
diff --git a/src/lib/collabmap.js b/src/lib/collabmap.js
index 6315f56b..d651e584 100644
--- a/src/lib/collabmap.js
+++ b/src/lib/collabmap.js
@@ -10,7 +10,7 @@
 // lives in worldmap.js, shared with the conference map.
 
 import collaborators from '/config/collaborators.json';
-import places from '/config/places.json';
+import { locate } from './places.js';
 import { map, project, toXY, spread, hueFor, KM_PER_UNIT, MAX_DRIFT_MILES, MAX_PIN_DRIFT }
   from './worldmap.js';
 import { byType, venueUrl, links, assists, primaryLink } from './data.js';
@@ -83,7 +83,7 @@ export function collaboratorMap() {
 
   for (const person of collaborators.people) {
     const posts = person.affiliations
-      .filter((a) => a.start && a.organization && places.places[a.organization])
+      .filter((a) => a.start && a.organization && locate(a.organization))
       // Newest first: what someone is doing now is the more useful fact, and it
       // is the order every other dated list on this site uses. The trajectory
       // is a line through the same points either way.
@@ -92,7 +92,7 @@ export function collaboratorMap() {
 
     const mine = papers.get(person.orcid) ?? [];
     const pins = posts.map((post) => {
-      const { lat, lon } = places.places[post.organization];
+      const { lat, lon } = locate(post.organization);
       const [x, y] = toXY(lon, lat);
       return {
         organization: post.organization,
diff --git a/src/lib/confmap.js b/src/lib/confmap.js
index bbacdc83..55f76d3a 100644
--- a/src/lib/confmap.js
+++ b/src/lib/confmap.js
@@ -11,7 +11,7 @@
 // Geometry is resolved here rather than in the component, so the page renders
 // plain numbers and the maths is testable without a DOM.
 
-import places from '/config/places.json';
+import { locate } from './places.js';
 import { map, toXY, spread, hueFor } from './worldmap.js';
 import { byType, dateLabel, links } from './data.js';
 
@@ -32,7 +32,7 @@ export const ONLINE = 'Online';
 export const ONLINE_AT = { lat: -70, lon: 0 };
 
 /** Where a location string is on Earth, or nothing if it is not settled. */
-const coords = (loc) => (loc === ONLINE ? ONLINE_AT : places.places[loc]);
+const coords = (loc) => (loc === ONLINE ? ONLINE_AT : locate(loc));
 
 /** How the CV's `kind` reads in a sentence about one talk. */
 const KIND = {
diff --git a/src/lib/places.js b/src/lib/places.js
new file mode 100644
index 00000000..9d541e1b
--- /dev/null
+++ b/src/lib/places.js
@@ -0,0 +1,14 @@
+// Where a named place is on Earth.
+//
+// `config/places.json` holds each coordinate once. A name that is the same
+// campus as another — "University of Chicago" and "The University of Chicago",
+// SAO and the CfA — is written `{ "sameAs": "<other name>" }` instead of
+// repeating coordinates, so the two can never drift apart.
+
+import data from '/config/places.json';
+
+/** `{ lat, lon }` for a place name, or undefined when it is not placed. */
+export function locate(name) {
+  const p = data.places[name];
+  return p?.sameAs ? data.places[p.sameAs] : p;
+}
diff --git a/tests/places.test.js b/tests/places.test.js
new file mode 100644
index 00000000..4038bfba
--- /dev/null
+++ b/tests/places.test.js
@@ -0,0 +1,35 @@
+import { describe, expect, it } from 'vitest';
+import data from '../config/places.json';
+import { locate } from '../src/lib/places.js';
+
+const entries = Object.entries(data.places);
+
+describe('config/places.json', () => {
+  it('points every sameAs at a place with coordinates of its own', () => {
+    for (const [name, p] of entries.filter(([, e]) => e.sameAs)) {
+      const target = data.places[p.sameAs];
+      expect(target, `${name} -> ${p.sameAs}`).toBeDefined();
+      expect(target.sameAs, `${name} -> ${p.sameAs} is a chain`).toBeUndefined();
+      expect(typeof target.lat).toBe('number');
+    }
+  });
+
+  it('writes each coordinate once: the same campus is a sameAs, not a copy', () => {
+    const seen = new Map();
+    for (const [name, p] of entries.filter(([, e]) => !e.sameAs)) {
+      const key = `${p.lat},${p.lon}`;
+      expect(seen.get(key), `${name} repeats the coordinates of ${seen.get(key)}`).toBeUndefined();
+      seen.set(key, name);
+    }
+  });
+
+  it('locate follows a sameAs and is undefined for an unplaced name', () => {
+    expect(locate('Smithsonian Astrophysical Observatory'))
+      .toBe(data.places['Center for Astrophysics Harvard & Smithsonian']);
+    expect(locate('Nowhere in particular')).toBeUndefined();
+  });
+
+  it('keeps the Smithsonian Institution in Washington, apart from the observatory', () => {
+    expect(locate('Smithsonian Institution').lat).not.toBe(locate('Smithsonian Astrophysical Observatory').lat);
+  });
+});