List Library - Message Deduplication
This document describes how the List Library handles duplicate messages.
- Duplicate posting: The same message sent to the same email list.
- Cross-posting: The same message sent to a different email list.
- Duplicate cross-posting: The same message sent to multiple email lists that have some or all of the same members.
List Library Configuration
The List Library lists are configured as follows:
- Do not allow duplicate posting
- Allow cross-posting
- Do not allow duplicate cross-posting
These settings result in the following email list behavior:
- Disallowing duplicate postings ensures that the same message doesn’t go out to a list more than once in a 24-hour period.
- Allowing cross-posting ensures that a message sent to multiple lists throughout the day, will be correctly sent to recipients on all the lists.
- Disallowing duplicate cross-posting ensures that if a person is on List A and List B, and a message is sent to both within a 24 hour period, that person will only receive one copy of the message.
All of these settings only apply to messages sent in the last 24 hours. Also note that these settings will not apply to a message that is sent to multiple lists at the same time, i.e. multiple list addresses in the to field. The List Library must process each message separately to know to which lists and which members it has been sent.
- Mary creates a message and addresses it to firstname.lastname@example.org and email@example.com. The message goes to both lists. If Bob is on both email lists, he will get the message twice.
- Mary creates a message and addresses it to firstname.lastname@example.org, waits 5 minutes, and then sends it to email@example.com. The message goes to both lists. If Bob is on both email lists, he will only get the message once because the List Library knows he already got the message from firstname.lastname@example.org.
- Mary creates a message and addresses it to email@example.com. There is a lot of traffic that day and Mary doesn’t get a copy of her message right away, so she sends the same message to firstname.lastname@example.org again. Only one message goes out, and the second is rejected as a duplicate.