Considerations regarding re-migrating the content of a SharePoint List to a different target List (4380670)

During a SharePoint migration using On Demand Migration for SharePoint (ODMSP), you initially migrated content to one library / list in the target and later remigrated to a different library / list within the same target site. Unexpectedly, folders and their contents - including newly added files - were moved from the original target library to the new one. Additionally, some root-level files failed to migrate, resulting in errors. 

• ODMSP preserves unique IDs of files and folders from the source during migration.
• Microsoft Import API enforces uniqueness of IDs across a site, allowing only one item with a given ID.
• When remigrating to a new library using the same IDs:
  • The Import API moves folders (not copies) from the old library to the new one if the folder ID already exists.
  • This includes all contents, even newly added files post-migration.
  • Root-level files with duplicate IDs fail to migrate, triggering a report entry that a file with the same ID already exists.

This behavior is by design in the Microsoft Import API.