When the mailbox is selected to be migrated again, either by running the same plan again or adding the mailbox to a new plan. The migration engine checks the source against the target and only migrates missing items.
NOTE: ODME will look in the folder and the trash for the email being migrated.
If any messages or folders change location in the source or the target mailbox and then they will be migrated again. Which will lead to duplicates in the target mailbox.
NOTE: The tool can not update previously migrated data with updated information.
To prevent this from occurring, the source and the target message/folder structure must remain unchanged between migrations.
This is important when migrating calendar items as it is updated more often than email. Attached is a scenario from Google to Office 365.