I lean about from https://help.zoho.com/portal/en/kb/crm/manage-crm-data/duplication-management/articles/check-duplicate-records#Mark_a_field_as_unique that a field can be marked as unique and then do duplicate checking for a module.

But I cannot find that in my account. The below screenshot is what i saw in my account:

Did I miss something? Or that feature is not available in free account?