diff --git a/apps/web/lib/entity-id.test.ts b/apps/web/lib/entity-id.test.ts
new file mode 100644
index 0000000..cd9d7fe
--- /dev/null
+++ b/apps/web/lib/entity-id.test.ts
@@ -0,0 +1,27 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import { createEntityId, dedupeEntityIds } from './entity-id';
+
+test('createEntityId stays unique within the same millisecond', () => {
+  const ids = Array.from({ length: 20 }, () => createEntityId('task', 1782361373857));
+
+  assert.equal(new Set(ids).size, ids.length);
+});
+
+test('dedupeEntityIds rewrites duplicate ids while preserving the first item id', () => {
+  const result = dedupeEntityIds(
+    [
+      { id: 'task-1', title: 'A' },
+      { id: 'task-1', title: 'B' },
+      { id: 'task-2', title: 'C' },
+      { id: 'task-1', title: 'D' },
+    ],
+    'task',
+    1782361373857,
+  );
+
+  assert.equal(result.changed, true);
+  assert.equal(result.items[0].id, 'task-1');
+  assert.equal(new Set(result.items.map((item) => item.id)).size, result.items.length);
+});
diff --git a/apps/web/lib/entity-id.ts b/apps/web/lib/entity-id.ts
new file mode 100644
index 0000000..7f9a9f4
--- /dev/null
+++ b/apps/web/lib/entity-id.ts
@@ -0,0 +1,43 @@
+let lastTimestamp = 0;
+let sameTimestampSequence = 0;
+
+export interface DedupeEntityIdsResult<T> {
+  items: T[];
+  changed: boolean;
+}
+
+export function createEntityId(prefix: string, timestamp = Date.now()): string {
+  if (timestamp === lastTimestamp) {
+    sameTimestampSequence += 1;
+  } else {
+    lastTimestamp = timestamp;
+    sameTimestampSequence = 0;
+  }
+  return `${prefix}-${timestamp}-${sameTimestampSequence.toString(36)}`;
+}
+
+export function dedupeEntityIds<T extends { id: string }>(
+  items: T[],
+  prefix: string,
+  timestamp = Date.now(),
+): DedupeEntityIdsResult<T> {
+  const seen = new Set<string>();
+  let changed = false;
+
+  const deduped = items.map((item) => {
+    if (item.id && !seen.has(item.id)) {
+      seen.add(item.id);
+      return item;
+    }
+
+    changed = true;
+    let nextId = createEntityId(prefix, timestamp);
+    while (seen.has(nextId)) {
+      nextId = createEntityId(prefix, timestamp);
+    }
+    seen.add(nextId);
+    return { ...item, id: nextId };
+  });
+
+  return { items: changed ? deduped : items, changed };
+}
diff --git a/apps/web/stores/useDevTaskStore.ts b/apps/web/stores/useDevTaskStore.ts
index 3a20535..3dfb9a7 100644
--- a/apps/web/stores/useDevTaskStore.ts
+++ b/apps/web/stores/useDevTaskStore.ts
@@ -3,6 +3,7 @@ import { create } from 'zustand';
 import type { DevTask, DevTaskStatus } from '@/lib/dev-task';
 import { generateTaskNo, isLegacyTask } from '@/lib/dev-task';
 import { applyDevTaskTransition, normalizeDevTaskOnCreate } from '@/lib/dev-task-workflow';
+import { createEntityId, dedupeEntityIds } from '@/lib/entity-id';
 import { loadServerData, saveServerData } from '@/lib/server-data';
 
 function saveStored(items: DevTask[]) {
@@ -16,7 +17,9 @@ async function loadStored(): Promise<DevTask[] | null> {
       if (Array.isArray(parsed) && parsed.some(isLegacyTask)) {
         return null;
       }
-      return parsed;
+      const result = dedupeEntityIds(parsed, 'task');
+      if (result.changed) saveStored(result.items);
+      return result.items;
     }
   } catch {}
   return null;
@@ -48,7 +51,7 @@ export const useDevTaskStore = create<DevTaskState>((set, get) => ({
     const now = new Date().toISOString();
     const task: DevTask = normalizeDevTaskOnCreate({
       ...data,
-      id: `task-${Date.now()}`,
+      id: createEntityId('task'),
       taskNo: generateTaskNo(list),
       isBlocked: false,
       createdAt: now,
diff --git a/apps/web/stores/useTestCaseStore.ts b/apps/web/stores/useTestCaseStore.ts
index 80fba2c..f7397fb 100644
--- a/apps/web/stores/useTestCaseStore.ts
+++ b/apps/web/stores/useTestCaseStore.ts
@@ -3,6 +3,7 @@ import { create } from 'zustand';
 import type { TestCase, TestCaseStatus } from '@/lib/test-case';
 import { generateCaseNo, normalizeTestCases } from '@/lib/test-case';
 import { applyTestCaseTransition, normalizeTestCaseOnCreate } from '@/lib/test-case-workflow';
+import { createEntityId, dedupeEntityIds } from '@/lib/entity-id';
 import { loadServerData, saveServerData } from '@/lib/server-data';
 
 function saveStored(items: TestCase[]) {
@@ -33,7 +34,11 @@ export const useTestCaseStore = create<TestCaseState>((set, get) => ({
 
   fetchTestCases: async () => {
     const cached = await loadStored();
-    if (cached) set({ testCases: normalizeTestCases(cached) });
+    if (cached) {
+      const result = dedupeEntityIds(normalizeTestCases(cached), 'tc');
+      if (result.changed) saveStored(result.items);
+      set({ testCases: result.items });
+    }
   },
 
   createTestCase: (data) => {
@@ -41,7 +46,7 @@ export const useTestCaseStore = create<TestCaseState>((set, get) => ({
     const now = new Date().toISOString();
     const tc: TestCase = normalizeTestCaseOnCreate({
       ...data,
-      id: `tc-${Date.now()}`,
+      id: createEntityId('tc'),
       caseNo: generateCaseNo(list),
       status: 'pending',
       estimateHours: data.estimateHours ?? 0.5,