Why did Lob mail the same postcard twice?
The Lob help pages give 3 common reasons. The first to rule out: A request that makes a postcard is not protected against repeats on its own.
Is it broken, or just quiet?
A number can fall because fewer people came, or because something stopped counting them. Below are the causes the Lob help pages give, and how to check each one yourself.
What causes it?
A request that makes a postcard is not protected against repeats on its own.
In the Lob help:
`POST` requests, however, are not idempotent.
Managing mail settings, read 2026-11-12.Each request to make a postcard makes a new one, so the same request made more than once can make more than one postcard.
In the Lob help:
If you send the same `POST` request 5 times, you will create 5 resources, assuming none of those requests err.
Managing mail settings, read 2026-11-12.After a network error, it can be unclear how many postcards were made, so a retry can make an extra one.
In the Lob help:
If a network error occurs, there is no deterministic way to ensure the exact number of resources created.
Managing mail settings, read 2026-11-12.
How do I check it myself?
Check whether each request to Lob carries an idempotency key. Lob uses it so the same request does not make a second postcard.
In the Lob help:
To perform an idempotent `POST` request, you simply need to provide an additional `Idempotency-Key` header
Managing mail settings, read 2026-11-12.Check that retries reuse the same key as the first try, so a retry does not make a duplicate.
In the Lob help:
You can safely retry the same request with the same Idempotency Key and be assured that no duplicates are created
Managing mail settings, read 2026-11-12.
How would I know the day it starts?
- Feder reads your postcards sent from Lob each day and compares each day with your own history.
- It does not find duplicate postcards. It tells you when postcards sent go quiet or move outside their usual range.
Some numbers are too quiet to judge daily, such as a form filled in a few times a week. Until there is enough of your own history to judge a change, Feder says it is still learning and scores nothing, rather than guessing.
See what Feder reads from Lob, which feeds Reach. Feder only reads. It never changes anything in Lob.
What else goes wrong with these numbers?
- Why are my Lob postcards not being created?Lob
- Why haven't my Lob postcards been sent yet?Lob
- Why did Google Analytics stop collecting data?Google Analytics
- Why aren't my key events showing in Google Analytics?Google Analytics
- Why does Google Analytics show (not set)?Google Analytics
See every question.
Where do these answers come from?
Every cause and check on this page comes from the Lob help pages, quoted above with the date we read them. Last reviewed 2026-11-12. We read them again by 2027-11-12. If one changes, this page changes or comes down.