How does message deduplication work?

Message deduplication prevents accidentally sending duplicate emails to the same recipient. This can happen from application bugs, retry logic, or user error.

Deduplication methods:

Message-ID tracking: The unique Message-ID header identifies each email. Systems can reject or flag duplicates with the same ID.

Recipient + campaign tracking: Track which recipients have received each campaign. Block subsequent sends of the same campaign to the same address.

Content hashing: Hash message content and recipient; identical hashes within a time window indicate duplicates.

Idempotency keys: API-based sending may support idempotency keys. Repeat requests with the same key don't send additional messages.

When deduplication matters:

Retry logic in application code might resend on timeout even if the first attempt succeeded

User double-clicks send button in a UI

Integration errors cause duplicate API calls

List uploads accidentally include the same addresses multiple times

ESP handling:

Many ESPs deduplicate automatically within campaigns

API endpoints may support idempotency parameters

Dashboard warnings may flag potential duplicate content

Deduplication has time windows; the same message months later may be intentional (re-engagement campaign) and should send.