diff --git a/public/js/ar/detect.js b/public/js/ar/detect.js
index 3ab7bd5..59a75de 100644
--- a/public/js/ar/detect.js
+++ b/public/js/ar/detect.js
@@ -14,9 +14,25 @@ export function createDetector() {
  */
 export function detectMarkers(detector, imageData, knownIds) {
   const raw = detector.detect(imageData).filter(m => quadArea(m.corners) > 100);
-  const usable = (knownIds && knownIds.size) ? raw.filter(m => knownIds.has(m.id)) : [];
+
+  /* Deduplicate by id. Two quads in one frame can decode to the same id — a
+   * misread of a different crest under blur or glare. That is poison for the
+   * board solve: both quads are handed the SAME world corners, so solvePnP gets
+   * two contradictory constraints for one point set and the pose collapses
+   * (seen on device as reproj jumping to 5px+). Keep the largest quad, which is
+   * the closest and best-resolved reading. */
+  const best = new Map();
+  for (const m of raw) {
+    const area = quadArea(m.corners);
+    const prev = best.get(m.id);
+    if (!prev || area > prev.area) best.set(m.id, { m, area });
+  }
+  const uniq = [...best.values()].map(e => e.m);
+
+  const usable = (knownIds && knownIds.size) ? uniq.filter(m => knownIds.has(m.id)) : [];
   usable.rawCount = raw.length;
   usable.rawIds = raw.map(m => m.id);
+  usable.dupes = raw.length - uniq.length;
   return usable;
 }