In partition definition filegroup name appears twice, what does that mean?
A partition scheme works with a partition function.

you start by creating the function where you will define the range. then the partition scheme will tell you where to store the data according to that range.

Or, to say it in another way, that particular partition scheme is just stating that for values that fall under the first condition store it in the PRIMARY filegroup, and for values that fall into the the second condition use the PRIMARY FILEGROUP.

you'll sometime see it in examples as CREATE PARTITION ... ALL TO ([PRIMARY])

Hope this helps