The contact deduplication setting warns you about duplicate contacts that exist in multiple sequences. By default, you cannot add the same email address to the same sequence. At least one email step must be added to the sequence to trigger running the deduplication check. Dedupe checks only run once when contacts are added to a sequence.
You can adjust this in your Account Settings and clicking the Safety tab.
These are the four options for deduplication settings:
Off - Do not deduplicate
This setting turns off deduplication entirely. We will never warn you when a duplicate contact has been added to another sequence, if they are about to receive a message in another sequence and if they have ever received a message.
Low - Contact is actively being messaged
We will only warn you when another sequence has messages to send to the same email address. This will not trigger if the other sequence has no email sequence steps or is archived.
Medium - Contact is being messaged or has been messaged
Along with warning you if they are about to receive a message, we will also warn you when they have received a message ever from any of your sequences regardless if it's active or archived. If they have never received a message or not about to, you will not be warned.
Here's an example of when it's recommended to adjust the setting to Medium:
If a contact appears in multiple sequences, they can be messaged again if the deduplication setting is set to low and the contact has replied in a different sequence. In this instance, it's best to select Medium instead of Low to prevent the contact from being messaged again in another sequence.
High - Contact exists in another sequence
The most extreme setting of them all. We'll warn you whenever a contact has ever been added to any other sequence, even if it has been archived.
Note: Deduplication settings can also be adjusted at the team level.