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Use import contacts to add contacts to a list
Please use Import contact instead of this call to add a contact to one or more lists.

This operation adds a contact to a given address book. The contact can be new or existing. If the contact already exists, then any data provided in your request that is already held for the contact will be overwritten and updated in the system.

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Are you importing multiple contacts?
If you're looking to import or update multiple contacts to an address book at once, then we highly recommend using Bulk import contacts in address book with merge option instead.
It allows you to import a file and has the benefit of being a single API call only, making it far less likely that your account will exceed its API calls per hour limit.