It appears that bpduplicate will duplicate images multiple times if they span tapes using the -id flag (duplicate duplicates, if you will).
Here is some (heavily edited) proof. First I list the contents of two media, only looking for copy 3. Each backupid here has 3 copies, the 1st two of which are on disk. The full listing is less pleasant to look at. You'll have to take my word for the fact that each physical tape only has the listed fragments.
You can see there are spanned images - host1_1354984347 starts on LT0001, then continues on LT0002. host2_1354970317 starts on LT0003 (not listed) then continues on LT0001. host4_1354981606 starts on LT0004, goes on to LT0005, then LT0002, and finishes on LT0006 (only LT0002 is listed).
If I try to duplicate LT0001, bpduplicate wants to grab host2_1354970317, whose 1st fragment is on LT0003 - showing that it's going to get images that don't start on the specified media, which I suspected earlier. Then it's going to grab host3_1354974745, which is wholly contained on LT0001, then it will get host1_1354984347, whose first fragment is on LT0001, then spans to LT0002 - which shows that it will get spanned images (which we all suspected).
If I try to duplicate LT0002, it again gets host1_1354984347, even though its first fragment is in LT0001 (and the entire image was duplicated when I did LT0001). Then it gets host4_1354981606, whose fragments span 4 tapes, but only the 4th and 5th or on the specified media.
Let me know if you agree with my analysis....
Bill
$ bpimmedia -mediaid LT0001 | egrep "IMAGE|FRAG 3"
IMAGE host1 8 host1_1354984347
FRAG 3 1 LT0001
FRAG 3 2 LT0001
FRAG 3 3 LT0002
IMAGE host2 8 host2_1354970317
FRAG 3 1 LT0003
FRAG 3 2 LT0001
IMAGE host3 8 host3_1354974745
FRAG 3 1 LT0001
$ bpimmedia -mediaid LT0002 | egrep "IMAGE|FRAG 3"
IMAGE host1 8 host1_1354984347
FRAG 3 1 LT0001
FRAG 3 2 LT0001
FRAG 3 3 LT0002
IMAGE host4 8 host4_1354981606
FRAG 3 1 LT0004
FRAG 3 2 LT0005
FRAG 3 3 LT0005
FRAG 3 4 LT0002
FRAG 3 5 LT0002
FRAG 3 6 LT0006
$ bpduplicate -p -id LT0001 -s 12/7/2012 -cn 3
INF - Skipping copy 1 of backup id host1_1354984347, is not required copy 3.
INF - Skipping copy 2 of backup id host1_1354984347, is not required copy 3.
INF - Skipping copy 1 of backup id host2_1354970317, is not required copy 3.
INF - Skipping copy 2 of backup id host2_1354970317, is not required copy 3.
INF - Skipping copy 1 of backup id host3_1354974745, is not required copy 3.
INF - Skipping copy 2 of backup id host3_1354974745, is not required copy 3.
Media id = LT0003 Server = host5
Bid = host2_1354970317 Kbytes = 247849728 Filenum = 48 Fragment = 1
Media id = LT0001 Server = host5
Bid = host2_1354970317 Kbytes = 474169419 Filenum = 1 Fragment = 2
Bid = host3_1354974745 Kbytes = 57230457 Filenum = 2 Fragment = 1
Bid = host1_1354984347 Kbytes = 1048575232 Filenum = 3 Fragment = 1
Bid = host1_1354984347 Kbytes = 96023808 Filenum = 4 Fragment = 2
Media id = LT0002 Server = host5
Bid = host1_1354984347 Kbytes = 429724192 Filenum = 1 Fragment = 3
$ bpduplicate -p -id LT0002 -s 12/7/2012 -cn 3
INF - Skipping copy 1 of backup id host1_1354984347, is not required copy 3.
INF - Skipping copy 2 of backup id host1_1354984347, is not required copy 3.
INF - Skipping copy 1 of backup id host4_1354981606, is not required copy 3.
INF - Skipping copy 2 of backup id host4_1354981606, is not required copy 3.
Media id = LT0001 Server = host5
Bid = host1_1354984347 Kbytes = 1048575232 Filenum = 3 Fragment = 1
Bid = host1_1354984347 Kbytes = 96023808 Filenum = 4 Fragment = 2
Media id = LT0002 Server = host5
Bid = host1_1354984347 Kbytes = 429724192 Filenum = 1 Fragment = 3
Bid = host4_1354981606 Kbytes = 1048575232 Filenum = 2 Fragment = 4
Bid = host4_1354981606 Kbytes = 295219712 Filenum = 3 Fragment = 5
Media id = LT0006 Server = host1
Bid = host4_1354981606 Kbytes = 660275744 Filenum = 3 Fragment = 6
Media id = LT0004 Server = host1
Bid = host4_1354981606 Kbytes = 468236032 Filenum = 5 Fragment = 1
Media id = LT0005 Server = host1
Bid = host4_1354981606 Kbytes = 1048575232 Filenum = 3 Fragment = 2
Bid = host4_1354981606 Kbytes = 697864448 Filenum = 4 Fragment = 3