The same commercial in different shows may share the same blocks even more than the shows.
Having worked on some deduplication systems, I would be _REALLY_ shocked if anyone's deduplication system can find similar video segments from two different shows and deduplicate them.
Unless the raw footage is being decoded and stored there are two problems. First even if the encoding of the video data results in identical P/B frames/slices it will probably exist at different offsets. Many of the block deduplication systems won't re-align block data. Aka the start of a P frame in one stream may be at offset X and in another stream its at offset Y, so even though the data may be identical for long periods of time the deduplication system won't detect it. The smarter systems might be able to detect small runs inside the frame sequence but even small changes in the transport metadata will probably screw this up. So, it would basically require a full blown video transport decoder to make this work properly. I'm not aware of any deduplication systems with video decoders, the ones with stream decoders like that tend to decode things like oracle databases.
Secondly, the commercials tend to be inserted in real time, with a real-time mpeg encoder. So even with a new I frame at the beginning of the commercial (doubtful), the state of the huffman coders/DCT heuristics/etc will cross over from the content into the commercials. Meaning that the compressed data is going to be completely different even though the video stream may look identical. Even if the data were decoded back to a raw uncompressed format its quite likely the frame data still won't match.
That said, it is possible with some frame matching heuristics to find identical sequences in different video streams. Hmmm, do I smell an automatic commercial removal tool?