When a user tries to subscribe to a newsletter where he/she is already subscribed, there are multiple conditions that can apply:

If the user subscribes with the same email address to the same email list, the contact will not be duplicated in your email list.

Yet, if the user adds other new data besides email address, for example, a new custom field, tag, or a different name. The contact details will be automatically updated.

If an existing subscriber subscribes to a different email list, the contact will be duplicated but won't be counted in your contact limit.

Learn about how duplicated contacts count in your contact limit.

When a user re-subscribers to your email list, they're opt-in will go through without notifying them.

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