個人用メール、予定表、連絡先管理に従来の Outlook on Windows を使用する
Item を生成するときに付与される Object ID でDuplicate の判断をしているのではないでしょうか。
There is no universal definition (operative word = "universal" otherwise described as "one size fits all) for a "duplicate" contact since different definitions can apply for different people depending on their scenario. Very specifically, not everyone considers two contact items with the same FirstName/Last/Name/EmailAddress as a duplicate since other factors may apply.