DataDomain takes special exceptions for data coming from CommVault to work around these issues. When you configure the DataDomain system, you tell it what kind of tape marker it should expect and it treats that data differently then normal backup data.
In addition, DataDomain is using Variable Length Segments to chunk up the data into bits that it can deduplicate. Overall, Variable Length Segment Deduplication systems get better deduplication rates than Fixed Length Segment systems. I don't know how ZFS does its deduplication, but it would be worth looking into.