This behavior is by design
Is the additional calendar is not needed, users need either to manually remove the shared calendars on the target after migration, or follow the steps below prior to migration to change the Sharing Permission Settings
Organization-wide
Under External Sharing options for primary calendars ensure that Share all information and Allow Managing of Calendars are not selected
Please refer to the On Demand Migration for Emails User Guide here, bullet point 7, for more details
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