When an CI is inserted in the cmdb_ci table or any of the child tables, the business rule "Create Asset on insert" is triggered. This business rule looks up the Asset>Administration>Asset-CI field mapping table and updates the records per the mapping.
Here if you have any mapping tied to the Asset itself rather than a field on the asset, you will see such duplication. We do not recommend mapping a field on asset or CI to the Asset or CI records directly. We suggest mapping them to a field on the asset/Ci record.