Can I merge Sharepoint lists with unique value conflicts by updating metadata?
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I am trying to merge or copy data from either an excel spreadsheet or another sharepoint list into a master list.  The main "item number" column will enforce unique values, but each item will be assigned to multiple "project numbers" using metadata.

Is it possible to tell sharepoint to auto-update the metadata when duplicate items are added to the list?  I could probably even edit the metadata manually if it would display the old entry for modification, but the only option I've seen is modify or delete the new item.