## TL;DR

Rules: generate one UUID per logical email (per recipient per event), store it with your send attempt, and reuse the same key on every retry of that attempt. Never reuse a key across different emails or you will suppress legitimate sends.

## Steps

1. Always send transactional emails with an idempotency key - const resp = await loops.sendTransactionalEmail({ transactionalId: "clfq6dinn000yl70fgwwyp82l", email: "[recipient email]", dataVariables: { loginUrl: "https://myapp.com/login/" }, headers: { "Idempotency-Key": "550e8400-e29b-41d4-a716-446655440000", }, }); Why: transactional sends are the calls you retry, and email has no undo. If your request times out after Loops accepted it, a plain retry sends the email twice. With Idempotency-Key, the retry is recognized as the same logical send. Rules: generate one UUID per logical email (per recipient per event), store it with your send attempt, and reuse the same key on every retry of that attempt. Never reuse a key across different emails or you will suppress legitimate sends.

## When to use

You are seeing this: Always send transactional emails with an idempotency key - const resp = await loops.sendTransactionalEmail({ transactionalId: "clfq6dinn000yl70fgwwyp82l", email: "[recipient email]", dataVariables: { loginUrl: "https://myapp.com/login/" }, headers: {... Use this skill when you run into "Loops sendTransactionalEmail supports Idempotency-Key - use it or retries double-send".

## When not to use

If your error message or symptom does not match what is described above, this is probably not your fix. Search for your exact error text instead of forcing this one to fit.

## Versions

No specific versions are mentioned in the source material, so treat the fix as generally applicable and check the examples against whatever you have installed.

## Why this happens

The original report does not dig into a root cause. It documents the symptom and the fix that resolved it.

## Sources

- https://myapp.com/login/"