A migrated Mail box appears to have some duplicate emails in it. In Lotus Notes they were in the Sent folder
This will occur if the user has copied the email to another folder as well. If an email is copied and not moved to another folder a duplicate email is created. This copied email is unique as it will have its own UNID. This Unique ID is what is used by NME to migrate a document.
As the two documents have separate NoteIDs even if they are identical in content, they will be migrated using NME.
To resolve this issue it is best not to Copy emails to other folders in Lotus Notes. Moving to a relevant folder is the preferred.
This could occur for any folder and not not restricted to the Sent folder.