When setting up the Duplicate backup job which option did you select when setting up the "New job to duplicate backup data"? If you selected the option to "Duplicate backup sets following a job." this should allow you to select the job to duplicate and set the destination to another location (ie. B2D or Tape source). There isn't many other options to set as it is simply a basic task.
Is the job failing or does the job appear to be continuing for the 20+ hours with no apparent end in sight? Also does the 1st job backup to the media server or is it on a separate node? If it is on a separate node then try restarting the Remote Agent if you haven't already done so, could possibly be a communication error between the media server and the remote agent. You might try running a small test job with a duplicate job as well to make sure it works properly.
Please note any Application event logs and/or error messages within BE... anything to help with resolving this issue.