I am migrating email from one email provider to another using Apple Mail. I want to make sure that I do not accidentally create duplicates as some emails are duplicated in various sources.

If I try to copy the same email from two different sources into the same folder, does Apple Mail prevent the duplicate from being stored?

If so, does anyone know how Apple Mail determines if the email message is a duplicate (same body, subject, from, to, etc)?

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    Define "duplicate" and someone might be able to answer that. If the messages are in two different places the headers are going to be different even if the message text is the same. May 5 at 0:59

Not only does Mail not prevent the presence of duplicate messages (2 messages with identical headers and contents, including time stamps), but it (or iCloud) sometimes creates duplicates (buggy behavior). These can be hidden in Mail.app but you will see them in iCloud-Mail (via Safari) or with another mail client such as Outlook.

If you need to delete duplicates you may want to have a look as https://www.macworld.com/article/2046576/how-to-delete-duplicate-email-messages.html .

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