1. A data processing apparatus comprising: a chunk store containing specimen data chunks of previously processed data sets, a chunk index containing a chunk identifier for at least one of said specimen data chunks, a discrete manifest store, containing at least one manifest that represents a corresponding one of the previously processed data sets and that comprises at least one reference to at least one of said specimen data chunks, at least one processor configured to: divide input data to be processed into input data chunks; generate a chunk identifier for at least a given one of said input data chunks; identify a chunk identifier in the chunk index corresponding to the generated chunk identifier; and identify, based on the identifying of the chunk identifier in the index corresponding to the generated chunk identifier, at least one manifest in the manifest store comprising at least one reference to a specimen data chunk identical to said given input data chunk.