windows 2003server seeing storage disk and mirror

I guess you have 2 HBA in your server, a couple of fabrics and your CX3-20. Each HBA can see your LUN but due to CX structure (and due to your SAN configuration), only a single path will be available at any time.

Are you running PowerPath on your host ??

yes

How many LUN's should you be seeing according to the Storage Group this host is in ? What are the HLU's (Host LUN numbers) of those LUN's ?
If it's only 1 in that Storage Group (and you're seeing 2) and the HLU is 1 or higher, the 2nd LUN you're seeing is the DGC LUN at HLU 0. Change your HLU to 0 and you should now only see the LUN once.

My guess is that it has nothing to do with PPath.

Hey ICE

Are you zoned to both SPA and SPB? if not, the problem caould be stemming from the fact that the LUN is owned by the SP that is not zoned. So for example, if LUN 0 is assigned to SPA, but you are zoned only to SPB, then although the host can see it, it wont be able to access it.

Typically these show up as unrecognized in disk manager. Powerpath should tresspass the LUN, however it is worth checking