Assuming this is policy based with duplicates set to follow the backups.
Are your media sets configured to make sure the original data is protected for long enough to be duplicated?
Did anything fail on previous jobs in the sequence
Be aware if a policy based duplicate is set to run immediately after the original backup, and the duplicate itself fails to complete the next time a duplicate runs it will be expecting to backup both previous backups not just the latest one.
For example
1st Weekly Backup to disk runs - Duplicate job then starts but fails (for example no available media)
2nd Weekly Backup to disk runs - Duplicate job starts and attempts to backup both 1st weekly data and 2nd weekly data
If you take that example and only protect the media set used by the weekly backup to disk jobs for 6 days, then the 2nd Weekly job will overwrite the 1st weekly job - but the duplicate will still expect both sets of data so will fail.
As you can see from above example to assist you we are likely to need details of when you run the backup to disk jobs, when you run the duplicate jobs, what your media sets are configured for and when previous jobs in the policy may have failed.
