I have a desktop with a SATA disk and SATA mode set to AHCI in the BIOS. I am running windows 7. When I run smartctl.exe --scan, the result returns two entries, 
/dev/sda -d ata # /dev/sda, ATA device
/dev/csmi0,0 -d ata # /dev/csmi0,0, ATA device

Both these entries are for the same disk i.e. their serial numbers match. 

Why does smartctl return two entries for the same disk?