Generate duplicate proofs for blocks with incorrectly chained merkle roots

[AshwinSekar]

Problem

#34787 adds the requirement that each shred of FEC set N should have a merkle root that is chained with the shred of FEC set N - 1.

In the rare case that all of FEC N's retransmitters are comprised, it is prudent to check the merkle root of shred N - 1 to make sure it is properly chained.