diff --git a/artifacts/api-server/src/routes/identity.ts b/artifacts/api-server/src/routes/identity.ts
index 66be46e..842d7dd 100644
--- a/artifacts/api-server/src/routes/identity.ts
+++ b/artifacts/api-server/src/routes/identity.ts
@@ -289,19 +289,49 @@ router.post("/identity/vouch", async (req: Request, res: Response) => {
     return;
   }
 
+  // ── Extract event ID for replay guard ────────────────────────────────────
+  const eventId = typeof ev["id"] === "string" ? ev["id"] : randomUUID();
+
   try {
     // Upsert vouchee identity so it exists before applying boost
     await trustService.getOrCreate(voucheePubkey);
 
-    await db.insert(nostrTrustVouches).values({
-      id: randomUUID(),
-      voucherPubkey: parsed.pubkey,
-      voucheePubkey,
-      vouchEventJson: JSON.stringify(event),
-      trustBoost: VOUCH_TRUST_BOOST,
-    });
+    // Idempotent insert — unique constraints on (voucher, vouchee) pair AND
+    // event_id prevent both duplicate vouches and event replay attacks.
+    // onConflictDoNothing().returning() returns [] on conflict: no boost applied.
+    const inserted = await db
+      .insert(nostrTrustVouches)
+      .values({
+        id: randomUUID(),
+        voucherPubkey: parsed.pubkey,
+        voucheePubkey,
+        eventId,
+        vouchEventJson: JSON.stringify(event),
+        trustBoost: VOUCH_TRUST_BOOST,
+      })
+      .onConflictDoNothing()
+      .returning({ id: nostrTrustVouches.id });
 
-    // Apply trust boost: read current score then write boosted value
+    if (inserted.length === 0) {
+      // Duplicate — either same pair already vouched or event replayed.
+      // Return existing state without applying another boost.
+      const voucheeIdentity = await trustService.getIdentityWithDecay(voucheePubkey);
+      logger.info("vouch duplicate — no additional boost applied", {
+        voucher: parsed.pubkey.slice(0, 8),
+        vouchee: voucheePubkey.slice(0, 8),
+      });
+      res.status(409).json({
+        ok: false,
+        duplicate: true,
+        message: "Already vouched for this identity — co-signature recorded but no additional boost applied",
+        voucheePubkey,
+        currentScore: voucheeIdentity?.trustScore,
+        currentTier: voucheeIdentity?.tier,
+      });
+      return;
+    }
+
+    // First-time vouch: apply the one-time trust boost
     const currentRows = await db
       .select({ score: nostrIdentities.trustScore })
       .from(nostrIdentities)
diff --git a/lib/db/migrations/0006_timmy_economic_peer.sql b/lib/db/migrations/0006_timmy_economic_peer.sql
index f1fa8b0..6abc8cd 100644
--- a/lib/db/migrations/0006_timmy_economic_peer.sql
+++ b/lib/db/migrations/0006_timmy_economic_peer.sql
@@ -22,18 +22,26 @@ CREATE INDEX IF NOT EXISTS idx_timmy_nostr_events_recipient
   ON timmy_nostr_events(recipient_pubkey);
 
 -- ── nostr_trust_vouches ───────────────────────────────────────────────────────
--- Records elite-tier co-signing events. Each row grants a trust boost to the
--- vouchee and stores the raw Nostr event for auditability.
+-- Records elite-tier co-signing events. Each row grants a one-time trust boost
+-- to the vouchee. Two uniqueness guards prevent trust fraud:
+--   1. (voucher_pubkey, vouchee_pubkey) — one vouch per pair, ever.
+--   2. event_id — a specific signed event cannot be replayed.
 
 CREATE TABLE IF NOT EXISTS nostr_trust_vouches (
   id                TEXT PRIMARY KEY,
   voucher_pubkey    TEXT NOT NULL REFERENCES nostr_identities(pubkey),
   vouchee_pubkey    TEXT NOT NULL,
+  event_id          TEXT NOT NULL,
   vouch_event_json  TEXT NOT NULL,
   trust_boost       INTEGER NOT NULL DEFAULT 20,
-  created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW()
+  created_at        TIMESTAMPTZ NOT NULL DEFAULT NOW(),
+
+  CONSTRAINT uq_nostr_trust_vouches_event_id UNIQUE (event_id)
 );
 
+CREATE UNIQUE INDEX IF NOT EXISTS uq_nostr_trust_vouches_pair
+  ON nostr_trust_vouches(voucher_pubkey, vouchee_pubkey);
+
 CREATE INDEX IF NOT EXISTS idx_nostr_trust_vouches_voucher
   ON nostr_trust_vouches(voucher_pubkey);
 
diff --git a/lib/db/src/schema/nostr-trust-vouches.ts b/lib/db/src/schema/nostr-trust-vouches.ts
index 47acd69..76f1a4f 100644
--- a/lib/db/src/schema/nostr-trust-vouches.ts
+++ b/lib/db/src/schema/nostr-trust-vouches.ts
@@ -1,20 +1,38 @@
-import { pgTable, text, integer, timestamp } from "drizzle-orm/pg-core";
+import { pgTable, text, integer, timestamp, uniqueIndex } from "drizzle-orm/pg-core";
 import { nostrIdentities } from "./nostr-identities";
 
-export const nostrTrustVouches = pgTable("nostr_trust_vouches", {
-  id: text("id").primaryKey(),
+// ── nostr_trust_vouches ───────────────────────────────────────────────────────
+// One row per elite co-signing event. Two uniqueness guards prevent trust fraud:
+//   1. (voucher_pubkey, vouchee_pubkey) — each elite may vouch for a given target
+//      only once; subsequent calls return existing state without additional boost.
+//   2. event_id — a specific signed event cannot be replayed against different
+//      target pubkeys.
 
-  voucherPubkey: text("voucher_pubkey")
-    .notNull()
-    .references(() => nostrIdentities.pubkey),
+export const nostrTrustVouches = pgTable(
+  "nostr_trust_vouches",
+  {
+    id: text("id").primaryKey(),
 
-  voucheePubkey: text("vouchee_pubkey").notNull(),
+    voucherPubkey: text("voucher_pubkey")
+      .notNull()
+      .references(() => nostrIdentities.pubkey),
 
-  vouchEventJson: text("vouch_event_json").notNull(),
+    voucheePubkey: text("vouchee_pubkey").notNull(),
 
-  trustBoost: integer("trust_boost").notNull().default(20),
+    // NIP-01 event id (sha256 of serialised event) — prevents replay of the same
+    // signed event against a different voucheePubkey.
+    eventId: text("event_id").notNull().unique(),
 
-  createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
-});
+    vouchEventJson: text("vouch_event_json").notNull(),
+
+    trustBoost: integer("trust_boost").notNull().default(20),
+
+    createdAt: timestamp("created_at", { withTimezone: true }).defaultNow().notNull(),
+  },
+  (table) => [
+    // Enforce one vouch per (voucher, vouchee) pair — idempotency guard.
+    uniqueIndex("uq_nostr_trust_vouches_pair").on(table.voucherPubkey, table.voucheePubkey),
+  ],
+);
 
 export type NostrTrustVouch = typeof nostrTrustVouches.$inferSelect;