Today (2023 March 10) I am having memories of helping with bcp from one SQL Server 2008 R2 database to another of a 9.5 billion+ row table back in 2013. Yikes. bcp out - no need to think about uniqueifiers and error 666.
bcp in of 9.5 billion rows to a table with a clustered index and some NCIs on the other hand...
SELECT text FROM sys.messages WHERE message_id = 666 and language_id=1033
The maximum system-generated unique value for a duplicate group was exceeded for index with partition ID %I64d. Dropping and re-creating the index may resolve this; otherwise, use another clustering key.SQL 2005 kb 937533 is relevant. Table spool may only be able to handle 2,147,483,648 input rows before exhausting the available values for uniqueifier.
Here's a SQL Server 2008 example from the wayback machine.