A computer is being imported or added, but another record with the same hostname already exists in the database.
This tends to occur when a Network Discovery is ran on the same IP range that has already been previously scanned. When the record is first brought in, the IP address is in the host name field until host name resolution is successful. If the IP ends up resolving to a host name already present in the database, this error is thrown.
You may simply right-click and delete the duplicate record with the error, or identify the conflicting record before deleting by adding the Host Name column to PDQ Inventory using the "Customize this grid" button found in the top left corner of the main computer window.