Hi Kashif,
Hi Pooja,
Thank you for writing to us.
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I am afraid there is no way to avoid duplicate mails in a pst, as upon each message addition, a new EntryID is assigned to the message.
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For adding messages to a mail, you can refer to:
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How to Add and save attachments using MailMessage
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Managing attachments with Outlook using MapiMessage
Please let us know if we can be of any of any additional help to you in this regard.