diff --git a/__tests__/edges-unique.test.ts b/__tests__/edges-unique.test.ts
new file mode 100644
index 00000000..49eced53
--- /dev/null
+++ b/__tests__/edges-unique.test.ts
@@ -0,0 +1,166 @@
+/**
+ * Edge Uniqueness Tests
+ *
+ * Regression tests for the bug where `INSERT OR IGNORE INTO edges` was
+ * silently a no-op: the only candidate key was the AUTOINCREMENT id (which
+ * never conflicts), so duplicate edges accumulated on every re-emission /
+ * re-resolution.
+ *
+ * Fix: a UNIQUE index on (source, target, kind, COALESCE(line, -1),
+ * COALESCE(col, -1)) backs a fresh-install schema and is also applied via
+ * migration v4 (with a dedup pass over existing rows).
+ */
+
+import { describe, it, expect, beforeEach, afterEach } from 'vitest';
+import * as fs from 'fs';
+import * as path from 'path';
+import * as os from 'os';
+import { DatabaseConnection } from '../src/db';
+import { QueryBuilder } from '../src/db/queries';
+import { Edge, Node } from '../src/types';
+import { runMigrations, getCurrentVersion, CURRENT_SCHEMA_VERSION } from '../src/db/migrations';
+
+function tempDb(): { dir: string; db: DatabaseConnection; q: QueryBuilder } {
+  const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-edges-unique-'));
+  const db = DatabaseConnection.initialize(path.join(dir, 'test.db'));
+  const q = new QueryBuilder(db.getDb());
+  return { dir, db, q };
+}
+
+function cleanup(dir: string, db: DatabaseConnection) {
+  db.close();
+  if (fs.existsSync(dir)) fs.rmSync(dir, { recursive: true, force: true });
+}
+
+function makeNode(id: string, name: string): Node {
+  return {
+    id,
+    kind: 'function',
+    name,
+    qualifiedName: `f::${name}`,
+    filePath: 'a.ts',
+    language: 'typescript',
+    startLine: 1,
+    endLine: 1,
+    startColumn: 0,
+    endColumn: 0,
+    updatedAt: Date.now(),
+  };
+}
+
+function edgesCount(db: DatabaseConnection): number {
+  const row = db.getDb().prepare('SELECT COUNT(*) as c FROM edges').get() as { c: number };
+  return row.c;
+}
+
+describe('Edge UNIQUE constraint (bug #2)', () => {
+  let dir: string;
+  let db: DatabaseConnection;
+  let q: QueryBuilder;
+
+  beforeEach(() => {
+    ({ dir, db, q } = tempDb());
+    q.insertNodes([makeNode('n1', 'foo'), makeNode('n2', 'bar')]);
+  });
+
+  afterEach(() => cleanup(dir, db));
+
+  it('rejects duplicate (source, target, kind, line, col)', () => {
+    const e: Edge = { source: 'n1', target: 'n2', kind: 'calls', line: 10, column: 5 };
+    q.insertEdge(e);
+    q.insertEdge(e); // INSERT OR IGNORE — should be a no-op now
+    expect(edgesCount(db)).toBe(1);
+  });
+
+  it('treats two NULL line edges as duplicates (COALESCE in unique index)', () => {
+    const e: Edge = { source: 'n1', target: 'n2', kind: 'calls' };
+    q.insertEdge(e);
+    q.insertEdge(e);
+    expect(edgesCount(db)).toBe(1);
+  });
+
+  it('allows same source/target/kind on different lines', () => {
+    q.insertEdge({ source: 'n1', target: 'n2', kind: 'calls', line: 1 });
+    q.insertEdge({ source: 'n1', target: 'n2', kind: 'calls', line: 2 });
+    expect(edgesCount(db)).toBe(2);
+  });
+
+  it('allows same source/target/line on different kinds', () => {
+    q.insertEdge({ source: 'n1', target: 'n2', kind: 'calls', line: 1 });
+    q.insertEdge({ source: 'n1', target: 'n2', kind: 'references', line: 1 });
+    expect(edgesCount(db)).toBe(2);
+  });
+
+  it('insertEdges (batch) dedupes within the same call', () => {
+    const e: Edge = { source: 'n1', target: 'n2', kind: 'calls', line: 1, column: 1 };
+    q.insertEdges([e, e, e]);
+    expect(edgesCount(db)).toBe(1);
+  });
+
+  it('survives the same edge being re-emitted across many cycles', () => {
+    const e: Edge = { source: 'n1', target: 'n2', kind: 'calls', line: 1 };
+    for (let i = 0; i < 100; i++) {
+      q.insertEdge(e);
+    }
+    expect(edgesCount(db)).toBe(1);
+  });
+});
+
+describe('Migration v4: dedup existing edges', () => {
+  let dir: string;
+  let dbPath: string;
+
+  beforeEach(() => {
+    dir = fs.mkdtempSync(path.join(os.tmpdir(), 'codegraph-migr-v4-'));
+    dbPath = path.join(dir, 'test.db');
+  });
+
+  afterEach(() => {
+    if (fs.existsSync(dir)) fs.rmSync(dir, { recursive: true, force: true });
+  });
+
+  it('collapses pre-existing duplicates and adds the UNIQUE index', () => {
+    // Build a v3-shaped database manually: schema, but simulate a stale
+    // version row + insert duplicates that the missing UNIQUE index let
+    // through. We use the real initialize() path then drop the index +
+    // version row to back-date the DB.
+    const db = DatabaseConnection.initialize(dbPath);
+    db.getDb().exec(`DROP INDEX IF EXISTS idx_edges_unique;`);
+    db.getDb().exec(`DELETE FROM schema_versions;`);
+    db.getDb().prepare(
+      'INSERT INTO schema_versions (version, applied_at, description) VALUES (3, ?, ?)'
+    ).run(Date.now(), 'simulated v3');
+
+    const q = new QueryBuilder(db.getDb());
+    q.insertNodes([makeNode('n1', 'foo'), makeNode('n2', 'bar')]);
+    // Force-insert duplicates via raw SQL (bypassing the constraint that
+    // is now absent). Three rows that should collapse to one.
+    const stmt = db.getDb().prepare(
+      'INSERT INTO edges (source, target, kind, line, col) VALUES (?, ?, ?, ?, ?)'
+    );
+    stmt.run('n1', 'n2', 'calls', 10, 5);
+    stmt.run('n1', 'n2', 'calls', 10, 5);
+    stmt.run('n1', 'n2', 'calls', 10, 5);
+    // And one with NULL line/col, also duplicated
+    stmt.run('n1', 'n2', 'references', null, null);
+    stmt.run('n1', 'n2', 'references', null, null);
+
+    expect(edgesCount(db)).toBe(5);
+    expect(getCurrentVersion(db.getDb())).toBe(3);
+
+    // Run migrations forward
+    runMigrations(db.getDb(), 3);
+
+    expect(getCurrentVersion(db.getDb())).toBe(CURRENT_SCHEMA_VERSION);
+    expect(CURRENT_SCHEMA_VERSION).toBeGreaterThanOrEqual(4);
+    // 3 calls dups → 1, 2 references dups → 1
+    expect(edgesCount(db)).toBe(2);
+
+    // Now the constraint is enforced: another duplicate insert is a no-op.
+    const q2 = new QueryBuilder(db.getDb());
+    q2.insertEdge({ source: 'n1', target: 'n2', kind: 'calls', line: 10, column: 5 });
+    expect(edgesCount(db)).toBe(2);
+
+    db.close();
+  });
+});
diff --git a/__tests__/foundation.test.ts b/__tests__/foundation.test.ts
index 9ee437da..4e8f204a 100644
--- a/__tests__/foundation.test.ts
+++ b/__tests__/foundation.test.ts
@@ -305,7 +305,7 @@ describe('Database Connection', () => {
 
     const version = db.getSchemaVersion();
     expect(version).not.toBeNull();
-    expect(version?.version).toBe(3);
+    expect(version?.version).toBe(4);
 
     db.close();
   });
diff --git a/__tests__/pr19-improvements.test.ts b/__tests__/pr19-improvements.test.ts
index 5fbe17d7..d43dceb2 100644
--- a/__tests__/pr19-improvements.test.ts
+++ b/__tests__/pr19-improvements.test.ts
@@ -299,7 +299,7 @@ describe('Best-Candidate Resolution', () => {
 describe('Schema v2 Migration', () => {
   it.skipIf(!HAS_SQLITE)('should have correct current schema version', async () => {
     const { CURRENT_SCHEMA_VERSION } = await import('../src/db/migrations');
-    expect(CURRENT_SCHEMA_VERSION).toBe(3);
+    expect(CURRENT_SCHEMA_VERSION).toBe(4);
   });
 
   it.skipIf(!HAS_SQLITE)('should have migration for version 2', async () => {
diff --git a/src/db/migrations.ts b/src/db/migrations.ts
index 0a256dbc..270265e2 100644
--- a/src/db/migrations.ts
+++ b/src/db/migrations.ts
@@ -9,7 +9,7 @@ import { SqliteDatabase } from './sqlite-adapter';
 /**
  * Current schema version
  */
-export const CURRENT_SCHEMA_VERSION = 3;
+export const CURRENT_SCHEMA_VERSION = 4;
 
 /**
  * Migration definition
@@ -54,6 +54,27 @@ const migrations: Migration[] = [
       `);
     },
   },
+  {
+    version: 4,
+    description: 'Dedup edges and enforce UNIQUE(source, target, kind, line, col) so INSERT OR IGNORE actually dedupes',
+    up: (db) => {
+      // Without a UNIQUE constraint the existing `INSERT OR IGNORE INTO
+      // edges` was a no-op for dedup purposes (the only candidate key was
+      // the AUTOINCREMENT id, which never conflicts). Existing databases
+      // can therefore contain accumulated duplicates from re-emission /
+      // re-resolution. Collapse those before adding the constraint, then
+      // create the UNIQUE index that future inserts will conflict against.
+      db.exec(`
+        DELETE FROM edges
+        WHERE id NOT IN (
+          SELECT MIN(id) FROM edges
+          GROUP BY source, target, kind, COALESCE(line, -1), COALESCE(col, -1)
+        );
+        CREATE UNIQUE INDEX IF NOT EXISTS idx_edges_unique
+          ON edges(source, target, kind, COALESCE(line, -1), COALESCE(col, -1));
+      `);
+    },
+  },
 ];
 
 /**
diff --git a/src/db/schema.sql b/src/db/schema.sql
index dd0a9f06..82c3e9b6 100644
--- a/src/db/schema.sql
+++ b/src/db/schema.sql
@@ -129,6 +129,15 @@ CREATE INDEX IF NOT EXISTS idx_edges_kind ON edges(kind);
 CREATE INDEX IF NOT EXISTS idx_edges_source_kind ON edges(source, kind);
 CREATE INDEX IF NOT EXISTS idx_edges_target_kind ON edges(target, kind);
 
+-- Uniqueness for (source, target, kind, line, col). The id column is an
+-- AUTOINCREMENT primary key, so without this index `INSERT OR IGNORE`
+-- would never see a conflict — duplicate edges would silently accumulate
+-- on every re-resolution / re-emission. COALESCE keeps two NULL line/col
+-- values comparable as equal (SQLite treats raw NULLs in a UNIQUE index
+-- as distinct).
+CREATE UNIQUE INDEX IF NOT EXISTS idx_edges_unique
+  ON edges(source, target, kind, COALESCE(line, -1), COALESCE(col, -1));
+
 -- File indexes
 CREATE INDEX IF NOT EXISTS idx_files_language ON files(language);
 CREATE INDEX IF NOT EXISTS idx_files_modified_at ON files(modified_at);