How to Prevent Duplicate Documents in Firestore

1import { doc, setDoc, serverTimestamp } from 'firebase/firestore'
2
3// Use email as document ID — guarantees one doc per email
4async function createUserProfile(email: string, displayName: string) {
5  const userRef = doc(db, 'user_profiles', email)
6
7  await setDoc(userRef, {
8    email,
9    displayName,
10    createdAt: serverTimestamp(),
11  })
12  // If called twice with the same email, the second call
13  // overwrites the first — no duplicate created
14}
15
16// Use a hash for composite uniqueness
17// e.g., prevent a user from joining the same group twice
18async function joinGroup(userId: string, groupId: string) {
19  const membershipId = `${userId}_${groupId}`
20  const ref = doc(db, 'memberships', membershipId)
21
22  await setDoc(ref, {
23    userId,
24    groupId,
25    joinedAt: serverTimestamp(),
26  })
27}

1import { doc, runTransaction, serverTimestamp } from 'firebase/firestore'
2
3async function createUniqueUsername(userId: string, username: string) {
4  const usernameRef = doc(db, 'usernames', username.toLowerCase())
5
6  await runTransaction(db, async (transaction) => {
7    const usernameDoc = await transaction.get(usernameRef)
8
9    if (usernameDoc.exists()) {
10      throw new Error(`Username "${username}" is already taken`)
11    }
12
13    // Create the username reservation
14    transaction.set(usernameRef, {
15      userId,
16      createdAt: serverTimestamp(),
17    })
18
19    // Also update the user's profile with the username
20    transaction.update(doc(db, 'users', userId), {
21      username: username.toLowerCase(),
22    })
23  })
24}

1// firestore.rules
2rules_version = '2';
3service cloud.firestore {
4  match /databases/{database}/documents {
5
6    // Username reservations: only allow create, never overwrite
7    match /usernames/{username} {
8      allow create: if request.auth != null
9        && resource == null                      // Document must not exist
10        && request.resource.data.userId == request.auth.uid; // Must be their own
11
12      allow read: if true;                       // Anyone can check availability
13
14      allow delete: if request.auth != null
15        && resource.data.userId == request.auth.uid; // Only owner can delete
16
17      allow update: if false;                    // Never update, only create/delete
18    }
19  }
20}

1import { doc, runTransaction, collection, serverTimestamp } from 'firebase/firestore'
2
3async function createUser(email: string, displayName: string) {
4  // Lookup collection: emails/{email} → { userId }
5  const emailRef = doc(db, 'emails', email.toLowerCase())
6
7  await runTransaction(db, async (transaction) => {
8    // Check if email is already registered
9    const emailDoc = await transaction.get(emailRef)
10    if (emailDoc.exists()) {
11      throw new Error('Email is already registered')
12    }
13
14    // Create the user document with auto-generated ID
15    const userRef = doc(collection(db, 'users'))
16
17    // Create both atomically
18    transaction.set(userRef, {
19      email: email.toLowerCase(),
20      displayName,
21      createdAt: serverTimestamp(),
22    })
23
24    transaction.set(emailRef, {
25      userId: userRef.id,
26      createdAt: serverTimestamp(),
27    })
28  })
29}
30
31// To check availability without creating:
32async function isEmailAvailable(email: string): Promise<boolean> {
33  const emailDoc = await getDoc(doc(db, 'emails', email.toLowerCase()))
34  return !emailDoc.exists()
35}

1import { doc, setDoc, serverTimestamp } from 'firebase/firestore'
2
3// Idempotent write: safe to execute multiple times
4async function submitVote(userId: string, pollId: string, choice: string) {
5  // Deterministic ID: one vote per user per poll
6  const voteId = `${userId}_${pollId}`
7  const voteRef = doc(db, 'votes', voteId)
8
9  // setDoc overwrites if offline duplicate syncs later
10  await setDoc(voteRef, {
11    userId,
12    pollId,
13    choice,
14    votedAt: serverTimestamp(),
15  })
16  // Even if called twice offline, only one vote document exists
17}