Technically if you are using Disk Storage for both Source and Destination (and no Hardware Deduplication) it would be best to use Deduplication on both Source and Destination.
The primary purpose of Deduplication/Compression is to minimize your overall capacity consumption at the Storage Layer by using references to associate similar block
If you enable the normal Deduplication option - this will create a local Deduplication Engine that is only used for that specific Storage Policy/Copy. As mentioned by Anthony if you use Global Deduplication you can associate multiple Storage Policy/Copy to this single Global Deduplication Policy, providing added space saving.
Further to above when you enable Deduplication on Source and Destination, Commvault will then levarage DASH (Deduplication Accelerate Streaming Hash) Copy, which provides added efficiency and security as only unique blocks are transferred across the network (with encryption) and any existing references, Commvault will increment the references in the Deduplication Database (created upon enabling Deduplication on the MediaAgent - preferably on SSD) further minimzing network bandwidth consumption and minimize additional write IOPs when writing to the Disk Library