How to ensure non-null duplicate values are not inserted in SQL Server 2005

I have a history database in SQL Server 2005 that is used to archive data from live database. I need to add a column in history database named RowID INT. This column cannot be made as identity since it need to store values from live database.

Note: In the live database the RowID is identity column. This column was added by me in live database also recently.

I need to make sure that in history database, the non-null values inserted into this column is unique. I understand that in SQL Server, we cannot make a nullable column as unique.

So, in SQL Server 2005, what is the most efficient way to make sure that insertion of non-null duplicate values into RowID columns throws an error?