diff --git a/backend/src/db/schema.sql b/backend/src/db/schema.sql
index dd49a26..0a40563 100644
--- a/backend/src/db/schema.sql
+++ b/backend/src/db/schema.sql
@@ -340,7 +340,13 @@ CREATE TABLE IF NOT EXISTS claims (
     last_reinforced_at  TIMESTAMPTZ NOT NULL DEFAULT NOW(),
     raw                 JSONB,
     created_at          TIMESTAMPTZ NOT NULL DEFAULT NOW(),
-    UNIQUE (subject_type, subject_id, predicate, object_type, object_id, source, user_id)
+    -- NULLS NOT DISTINCT (PG15+) is load-bearing: user_id is NULL for every
+    -- objective claim, and with default NULLS DISTINCT semantics the
+    -- ON CONFLICT clauses in upsertClaim() / MbSpineWriter never fire, so
+    -- re-enrichment inserts duplicates instead of reinforcing.
+    CONSTRAINT claims_edge_source_user_key
+      UNIQUE NULLS NOT DISTINCT
+      (subject_type, subject_id, predicate, object_type, object_id, source, user_id)
 );
 
 CREATE INDEX IF NOT EXISTS idx_claims_subject ON claims (subject_type, subject_id, predicate);
diff --git a/backend/src/services/db.service.ts b/backend/src/services/db.service.ts
index f7b2c3f..0da2109 100644
--- a/backend/src/services/db.service.ts
+++ b/backend/src/services/db.service.ts
@@ -504,6 +504,90 @@ const MIGRATIONS: { id: string; sql: string }[] = [
       JOIN artists a ON a.id = cf.object_id;
     `,
   },
+  {
+    id: '20260730_claims_dedup_nulls_not_distinct',
+    sql: `
+      -- The claims uniqueness constraint was declared as a plain
+      -- UNIQUE (subject_type, subject_id, predicate, object_type, object_id,
+      -- source, user_id). Every objective claim (MB, Discogs, tags) has
+      -- user_id IS NULL, and Postgres treats NULLs as distinct, so none of the
+      -- ON CONFLICT clauses in upsertClaim() / MbSpineWriter ever fired:
+      -- re-enrichment inserted a fresh duplicate row every time instead of
+      -- reinforcing. claim_fusion is SUM(trust * confidence * recency), so a
+      -- duplicated edge carried N times its intended weight.
+      --
+      -- Two phases, in this order (the constraint cannot be added while
+      -- duplicates exist):
+      --   1. collapse each duplicate group into its most recently reinforced
+      --      row, carrying forward MAX(last_reinforced_at) / MAX(evidence_at) /
+      --      MAX(confidence) so reinforcement recency is not lost;
+      --   2. replace the constraint with a NULLS NOT DISTINCT version (PG15+).
+
+      -- Phase 1: dedup.
+      CREATE TEMP TABLE claims_dedup ON COMMIT DROP AS
+      SELECT
+        id,
+        ROW_NUMBER() OVER ordered AS rn,
+        -- These MUST use the unordered window: an ORDER BY in the window spec
+        -- makes the default frame "UNBOUNDED PRECEDING TO CURRENT ROW", turning
+        -- MAX() into a running maximum rather than a per-group one.
+        MAX(last_reinforced_at) OVER grp AS max_last_reinforced_at,
+        MAX(evidence_at)        OVER grp AS max_evidence_at,
+        MAX(confidence)         OVER grp AS max_confidence
+      FROM claims
+      WINDOW
+        grp AS (
+          PARTITION BY subject_type, subject_id, predicate, object_type, object_id,
+                       source,
+                       COALESCE(user_id, '00000000-0000-0000-0000-000000000000'::uuid)
+        ),
+        ordered AS (
+          grp ORDER BY last_reinforced_at DESC, evidence_at DESC, id
+        );
+
+      -- Keeper of each group absorbs the group's best values.
+      UPDATE claims c
+      SET last_reinforced_at = d.max_last_reinforced_at,
+          evidence_at        = d.max_evidence_at,
+          confidence         = d.max_confidence
+      FROM claims_dedup d
+      WHERE c.id = d.id
+        AND d.rn = 1;
+
+      DELETE FROM claims c
+      USING claims_dedup d
+      WHERE c.id = d.id
+        AND d.rn > 1;
+
+      -- Phase 2: replace the constraint. The live DB has drifted from
+      -- schema.sql, so find the existing constraint by its definition rather
+      -- than assuming Postgres' auto-generated name.
+      DO $mig$
+      DECLARE
+        cname TEXT;
+      BEGIN
+        FOR cname IN
+          SELECT con.conname
+          FROM pg_constraint con
+          WHERE con.conrelid = 'claims'::regclass
+            AND con.contype = 'u'
+            AND pg_get_constraintdef(con.oid) LIKE '%subject_type%'
+            AND pg_get_constraintdef(con.oid) NOT LIKE '%NULLS NOT DISTINCT%'
+        LOOP
+          EXECUTE format('ALTER TABLE claims DROP CONSTRAINT %I', cname);
+        END LOOP;
+
+        IF NOT EXISTS (
+          SELECT 1 FROM pg_constraint
+          WHERE conrelid = 'claims'::regclass AND conname = 'claims_edge_source_user_key'
+        ) THEN
+          ALTER TABLE claims ADD CONSTRAINT claims_edge_source_user_key
+            UNIQUE NULLS NOT DISTINCT
+            (subject_type, subject_id, predicate, object_type, object_id, source, user_id);
+        END IF;
+      END $mig$;
+    `,
+  },
 ];
 
 export class DbService {