feat: near-duplicate campaign title detection with X-Allow-Duplicate override

Author: sudo-robi

backend/src/index.ts

@@ -26,6 +26,7 @@ import {
   CampaignStatus,
   claimCampaign,
   createCampaign,
+  findNearDuplicate,
   getCampaign,
   getCampaignWithProgress,
   getContributorSummary,
@@ -492,6 +493,19 @@ app.post(
       throw new AppError('deadline must be in the future.', 400, 'INVALID_DEADLINE');
     }
 
+    const allowDuplicate = req.headers['x-allow-duplicate'] === 'true';
+
+    if (!allowDuplicate) {
+      const duplicate = findNearDuplicate(body.title, body.creator);
+      if (duplicate) {
+        throw new AppError(
+          `Near-duplicate campaign found: "${duplicate.title}" (Levenshtein distance: ${duplicate.distance}). Use X-Allow-Duplicate: true header to override.`,
+          409,
+          'NEAR_DUPLICATE_CAMPAIGN',
+        );
+      }
+    }
+
     const campaignInput = {
       ...body,
       maxPerContributor:

backend/src/services/campaignStore.test.ts

@@ -22,6 +22,7 @@ let getPledges: CampaignStoreModule['getPledges'];
 let getDb: DbModule['getDb'];
 let getCampaignHistory: EventHistoryModule['getCampaignHistory'];
 let addPledge: CampaignStoreModule['addPledge'];
+let findNearDuplicate: CampaignStoreModule['findNearDuplicate'];
 
 const CREATOR = `G${'A'.repeat(55)}`;
 const CONTRIBUTOR = `G${'B'.repeat(55)}`;
@@ -41,6 +42,7 @@ beforeAll(async () => {
     getCampaign,
     getPledges,
     addPledge,
+    findNearDuplicate,
   } = await import('./campaignStore'));
   ({ getDb } = await import('./db'));
   ({ getCampaignHistory } = await import('./eventHistory'));
@@ -164,6 +166,73 @@ describe('on-chain pledge reconciliation', () => {
   });
 });
 
+describe('near-duplicate detection', () => {
+  const FUTURE_DEADLINE = Math.floor(Date.now() / 1000) + 86400;
+
+  it('returns duplicate when same creator creates near-identical title', () => {
+    createCampaign({
+      creator: CREATOR,
+      title: 'Help me build a rocket',
+      description: 'desc',
+      assetCode: 'USDC',
+      targetAmount: 100,
+      deadline: FUTURE_DEADLINE,
+    });
+
+    const result = findNearDuplicate('Help me build a rocket!!', CREATOR);
+    expect(result).toBeDefined();
+    expect(result!.distance).toBeLessThan(5);
+  });
+
+  it('allows same title from different creator', () => {
+    createCampaign({
+      creator: CREATOR,
+      title: 'Fund my project',
+      description: 'desc',
+      assetCode: 'USDC',
+      targetAmount: 100,
+      deadline: FUTURE_DEADLINE,
+    });
+
+    const otherCreator = `G${'D'.repeat(55)}`;
+    const result = findNearDuplicate('Fund my project', otherCreator);
+    expect(result).toBeUndefined();
+  });
+
+  it('returns undefined when title is sufficiently different', () => {
+    createCampaign({
+      creator: CREATOR,
+      title: 'Completely different topic',
+      description: 'desc',
+      assetCode: 'USDC',
+      targetAmount: 100,
+      deadline: FUTURE_DEADLINE,
+    });
+
+    const result = findNearDuplicate('Totally unrelated campaign', CREATOR);
+    expect(result).toBeUndefined();
+  });
+
+  it('ignores campaigns older than 24 hours', () => {
+    const db = getDb();
+    const oldTimestamp = Math.floor(Date.now() / 1000) - 60 * 60 * 25;
+
+    const campaign = createCampaign({
+      creator: CREATOR,
+      title: 'Old campaign title',
+      description: 'desc',
+      assetCode: 'USDC',
+      targetAmount: 100,
+      deadline: FUTURE_DEADLINE,
+    });
+
+    db.prepare(`UPDATE campaigns SET created_at = ? WHERE id = ?`).run(oldTimestamp, campaign.id);
+
+    const result = findNearDuplicate('Old campaign title', CREATOR);
+    expect(result).toBeUndefined();
+  });
+});
+
 describe('campaign pledge pagination', () => {
   it('returns pledges in reverse chronological order with pagination metadata inputs', () => {
     const futureDeadline = Math.floor(Date.now() / 1000) + 86400;

backend/src/services/campaignStore.ts

@@ -133,6 +133,51 @@ function round(value: number): number {
   return Number(value.toFixed(2));
 }
 
+function levenshtein(a: string, b: string): number {
+  const an = a.length;
+  const bn = b.length;
+  if (an === 0) return bn;
+  if (bn === 0) return an;
+  const matrix: number[] = new Array(bn + 1);
+  for (let i = 0; i <= bn; i++) matrix[i] = i;
+  for (let i = 1; i <= an; i++) {
+    let prev = i;
+    for (let j = 1; j <= bn; j++) {
+      const cost = a[i - 1] === b[j - 1] ? 0 : 1;
+      const val = Math.min(
+        prev + 1,
+        matrix[j] + 1,
+        matrix[j - 1] + cost,
+      );
+      matrix[j - 1] = prev;
+      prev = val;
+    }
+    matrix[bn] = prev;
+  }
+  return matrix[bn];
+}
+
+export function findNearDuplicate(
+  title: string,
+  creator: string,
+): { campaignId: string; title: string; distance: number } | undefined {
+  const db = getDb();
+  const twentyFourHoursAgo = nowInSeconds() - 60 * 60 * 24;
+  const rows = db
+    .prepare(
+      `SELECT id, title FROM campaigns WHERE creator = ? AND created_at >= ? AND deleted_at IS NULL`,
+    )
+    .all(creator, twentyFourHoursAgo) as Array<{ id: string; title: string }>;
+
+  for (const row of rows) {
+    const distance = levenshtein(title.trim().toLowerCase(), row.title.trim().toLowerCase());
+    if (distance < 5) {
+      return { campaignId: row.id, title: row.title, distance };
+    }
+  }
+  return undefined;
+}
+
 function rowToCampaign(row: CampaignRow): CampaignRecord {
   const acceptedTokens = JSON.parse(row.accepted_tokens_json);
   return {