diff --git a/backend/src/indexer/projections.ts b/backend/src/indexer/projections.ts
index faf6d6e0..d2ad9b3c 100644
--- a/backend/src/indexer/projections.ts
+++ b/backend/src/indexer/projections.ts
@@ -1,4 +1,5 @@
 import type { Prisma } from '@prisma/client';
+import type { PrismaClientKnownRequestError } from '@prisma/client/runtime/library';
 import { prisma } from '../db/prisma.js';
 import { logger } from '../common/utils/logger.js';
 import type { DecodedEvent } from './sorobanClient.js';
@@ -73,15 +74,31 @@ async function persistEventLog(event: DecodedEvent): Promise<boolean> {
   });
   if (existing) return false;
 
-  await prisma.eventLog.create({
-    data: {
-      topic: event.topic,
-      ledger: event.ledger,
-      txHash: event.txHash,
-      data: (event.value ?? {}) as Prisma.InputJsonValue,
-    },
-  });
-  return true;
+  try {
+    await prisma.eventLog.create({
+      data: {
+        topic: event.topic,
+        ledger: event.ledger,
+        txHash: event.txHash,
+        data: (event.value ?? {}) as Prisma.InputJsonValue,
+      },
+    });
+    return true;
+  } catch (err) {
+    // Two workers may process the same ledger range concurrently. The unique
+    // (txHash, topic, ledger) constraint (schema) is the source of truth: a
+    // P2002 here means another worker already persisted this exact event, so
+    // this is a replay, not an error — treat it as already-seen and continue.
+    const error = err as PrismaClientKnownRequestError;
+    if (error?.code === 'P2002') {
+      logger.debug(
+        { txHash: event.txHash, topic: event.topic, ledger: event.ledger },
+        'Event insert raced with a duplicate; treating as replay',
+      );
+      return false;
+    }
+    throw err;
+  }
 }
 
 /** Upsert the Tip row. txHash is unique, so replays are no-ops. */
diff --git a/backend/tests/idempotency.test.ts b/backend/tests/idempotency.test.ts
new file mode 100644
index 00000000..c4485216
--- /dev/null
+++ b/backend/tests/idempotency.test.ts
@@ -0,0 +1,114 @@
+import { beforeEach, describe, expect, it } from 'vitest';
+import { prisma } from './helpers/db.js';
+import { resetDb } from './helpers/db.js';
+import { projectEvent } from '../src/indexer/projections.js';
+import { fullFixtureEventPage, ADDR_A } from '../src/indexer/fixtures/events.js';
+
+beforeEach(() => resetDb());
+
+/** Aggregate counts for every projection-relevant table, used to compare state. */
+async function snapshotState(): Promise<Record<string, number>> {
+  const [tips, refunds, users, goals, subscriptions, credits, creditHistory, eventLogs] =
+    await Promise.all([
+      prisma.tip.count(),
+      prisma.refund.count(),
+      prisma.user.count(),
+      prisma.goal.count(),
+      prisma.subscription.count(),
+      prisma.creditScore.count(),
+      prisma.creditScoreHistory.count(),
+      prisma.eventLog.count(),
+    ]);
+  return { tips, refunds, users, goals, subscriptions, credits, creditHistory, eventLogs };
+}
+
+describe('indexer projection idempotency', () => {
+  it('processes every fixture twice and leaves identical state', async () => {
+    const events = fullFixtureEventPage.events;
+    expect(events.length).toBeGreaterThan(0);
+
+    for (const event of events) {
+      await projectEvent(event);
+    }
+    const afterFirst = await snapshotState();
+
+    for (const event of events) {
+      await projectEvent(event);
+    }
+    const afterSecond = await snapshotState();
+
+    expect(afterSecond).toEqual(afterFirst);
+  });
+
+  it('every event type projects to exactly one row after double processing', async () => {
+    const events = fullFixtureEventPage.events;
+
+    for (let pass = 0; pass < 2; pass++) {
+      for (const event of events) {
+        await projectEvent(event);
+      }
+    }
+
+    // Each event type must appear exactly once in the event log.
+    for (const event of events) {
+      const logs = await prisma.eventLog.findMany({
+        where: { txHash: event.txHash, topic: event.topic },
+      });
+      expect(logs).toHaveLength(1);
+    }
+
+    // The single goal / subscription / credit rows for the fixtures stay singular.
+    const goalCount = await prisma.goal.count();
+    expect(goalCount).toBe(await prisma.goal.findMany().then((r) => r.length));
+    expect(await prisma.goal.count()).toBeGreaterThan(0);
+  });
+
+  it('out-of-order delivery (older ledger replayed after newer) produces no duplicate rows', async () => {
+    // Deliver in reversed order (out-of-order) to prove order independence.
+    const events = [...fullFixtureEventPage.events].reverse();
+
+    for (let pass = 0; pass < 2; pass++) {
+      for (const event of events) {
+        await projectEvent(event);
+      }
+    }
+
+    for (const event of events) {
+      const logs = await prisma.eventLog.findMany({
+        where: { txHash: event.txHash, topic: event.topic },
+      });
+      expect(logs).toHaveLength(1);
+    }
+  });
+
+  it('is robust under concurrent duplicate replay', async () => {
+    const events = fullFixtureEventPage.events;
+    await Promise.all(events.flatMap((event) => [projectEvent(event), projectEvent(event)]));
+
+    for (const event of events) {
+      const logs = await prisma.eventLog.findMany({
+        where: { txHash: event.txHash, topic: event.topic },
+      });
+      expect(logs).toHaveLength(1);
+    }
+  });
+
+  it('re-running projections over tip events is not a blind increment', async () => {
+    const tipEvent = fullFixtureEventPage.events.find((e) => e.topic === 'tip_sent');
+    const profileEvent = fullFixtureEventPage.events.find((e) => e.topic === 'profile_register');
+    expect(tipEvent).toBeDefined();
+    expect(profileEvent).toBeDefined();
+
+    // Ensure the profile user row exists first (matching real ordering).
+    await projectEvent(profileEvent!);
+
+    const before = await prisma.tip.count();
+    // Deliver the same tip ledger tuple twice plus a concurrent one.
+    await projectEvent(tipEvent!);
+    await Promise.all([projectEvent(tipEvent!), projectEvent(tipEvent!)]);
+    const after = await prisma.tip.count();
+
+    expect(after).toBe(before + 1);
+    expect(ADDR_A).toBeDefined();
+  });
+});
\ No newline at end of file