Expired Exchange backup sets are not deleted as expected by DLM and remain on disk storage even if duplicate jobs have succeeded.
These expired Exchange backup sets are deleted when they are not parts of the latest recovery chain.
The following additional DLM grooming rule is applied to Exchange backup sets.
・The original backup set (full or incremental) on disk cannot be groomed if it has unexpired or retained dependents even if it has copies on disk or tape.
This is by design in DLM.
Refer to the document link under Related Articles to understand the general behavior of DLM.
Backup Exec 2012
DLM (Data Lifecycle Management)