Duplicate job running on wrong media server

We have a new installation of Netbackup 7.5 with a master server at one site (site A) and a media server at another site (site B).

We had a backup from site B that was on disk and wanted to duplicate this to tape.

The duplicate job is using the master server as the media server, so data is travelling across our WAN link, even though the data resides on disk at site B and is going to tape at the same location.

Is there a way I can force the duplicate job to use site B media server?

Thanks in advance.