Since the tape has no barcode you will need to tell
NetBackup what slot the tape is in. Since you used it
once already there should be a volume already created
for it. If you didn't move the tape to standalone,
NetBackup should already know where the tape is and
not exhibit the behavior you mentioned. If you moved
the tape to the standalone volume group, you will need
to move it back into the robot volume group and let
NetBackup know what slot the tape is in. If you don't
then NetBackup will treat the "unknown" tape as a new
one and try to give it a new volume. 

"A wise man once said 'Wherever you go, there you are'"
      -Mike Brady