Why is the same sd-card partition mounted on 7 different mount points?
what version of CM are you using? A bunch of those mount points don't even exist in my version of CM (CM7)

tmpfs                   173052        32    173020    0% /dev
tmpfs                   173052         0    173052    0% /mnt/asec
tmpfs                   173052         0    173052    0% /mnt/obb
/dev/block/mtdblock2    192000    146684     45316   76% /system  
/dev/block/mtdblock3     81920     35816     46104   44% /cache  
/dev/block/mtdblock5     16384     14512      1872   89% /radio  
/dev/block/mmcblk0p2   1935192    286276   1648916   15% /data  
/dev/block/mtdblock6    176128    156340     19788   89% /datadata  
/dev/block/mtdblock4     12800      6756      6044   53% /efs  
/dev/block/mmcblk1p2   1874206     32898   1741308    2% /sd-ext  
/dev/block/mmcblk0p2   1935192    286276   1648916   15% /cache/download  
/dev/block/mmcblk0p2   1935192    286276   1648916   15% /cache/download  
/dev/block/vold/179:1  13824672   4444320   9380352  32% /mnt/sdcard   
/dev/block/vold/179:1  13824672   4444320   9380352  32% /mnt/secure/asec  
/dev/block/vold/179:9  5869368    1714680   4154688  29% /mnt/emmc


There is what i get when i do a df. I did think it was interesting that /cache/download was in there twice though. But, also, if you notice, both /data and /cache/download are on the same device. Really, it is not uncommon that a device could have multiple mounts.