You can get the backupid and media id in the catalog in GUI but if you still want to do using cmd check the below commands. Make sure while doing bpexpdate of image do mention the copy number or else it won't work.
Also if you just run the command bpimmedia -disk -client <client_name> it is not populating the complete information and that is the reason you are seeing the same backup id, use the switch to get the complete information.
C:\Users\sdas>bpimmedia -disk -client linux.eur.xl -d 08/06/2012 -e 08/07/2012 -U
--------------------------------------------------------------------------------
Backup-ID: linux.eur.xl_1344301201
Policy: eudaix-dd-00amProd
Schedule Type: INCR
Retention Level 1
Number of Files: 160
Compression: N
Encryption: N
Image Type: Regular
Primary Copy: 1
Expires: INFINITY
Copy Number: 1
Fragment Number: 1
Fragment Size (KB): 69056
Media Type: Disk
Density: -
File Number: -
Offset: -
Host: delhi0005.eur.xl
Device Written On: -
MPX: N
Expires: INFINITY
Retention Level: 1
MediaID: @aaaa3
C:\Users\sdas>bpexpdate -backupid linux.eur.xl_1344301201 -d 08/15/2012 -
copy 1
Are you SURE you want to change linux.eur.xl_1344301201 copy 1
to expire on 8/15/2012 12:00:00 AM y/n (n)? y
C:\Users\sdas>bpimmedia -disk -client linux.eur.xl -d 08/06/2012 -e 08/07
/2012 -U
--------------------------------------------------------------------------------
Backup-ID: linux.eur.xl_1344301201
Policy: eudaix-dd-00amProd
Schedule Type: INCR
Retention Level 1
Number of Files: 160
Compression: N
Encryption: N
Image Type: Regular
Primary Copy: 1
Expires: 00:00 08/15/2012
Copy Number: 1
Fragment Number: 1
Fragment Size (KB): 69056
Media Type: Disk
Density: -
File Number: -
Offset: -
Host: delhi0005.eur.xl
Device Written On: -
MPX: N
Expires: 00:00 08/15/2012
Retention Level: 1
MediaID: @aaaa3
C:\Users\sdas>