diff --git a/infra/firestore/firestore.indexes.json b/infra/firestore/firestore.indexes.json
index 3a8216c..3308fb7 100644
--- a/infra/firestore/firestore.indexes.json
+++ b/infra/firestore/firestore.indexes.json
@@ -30,6 +30,15 @@
         { "fieldPath": "ended_at", "order": "ASCENDING" }
       ]
     },
+    {
+      "//": "c2-core internal/dedup.py:84 — system_id == X AND started_at within a +/- window. Was live-failing on essentially every inbound call (server-26#84): dedup caught the FAILED_PRECONDITION and degraded to \"not a duplicate\", so double-heard transmissions were stored twice and would have been transcribed and correlated twice the moment an AI window opened. Created directly on c2-server 2026-08-28.",
+      "collectionGroup": "calls",
+      "queryScope": "COLLECTION",
+      "fields": [
+        { "fieldPath": "system_id", "order": "ASCENDING" },
+        { "fieldPath": "started_at", "order": "ASCENDING" }
+      ]
+    },
     {
       "//": "c2-core internal/vocabulary_learner.py:290 — system_id == X AND ended_at >= cutoff. Backend only; was live but undeclared until 2026-08-23.",
       "collectionGroup": "calls",