Allocate random case number SQL 2008 different each time

Continuing to the query below in the link,

Allocate Unique Random case number SQL 2008

I need to run this every month and random case is allocated for each team member. I store this in a table and when it is run again next month, same case should not be allocated. Until 6 months, same case should not be allocated for each team member. Could you please let me know how do I achieve this. Thank you.