Final answer:
Dupin's conclusions focus on the psychological elements of a crime, with the importance of a document's ready availability or its potential destruction being crucial. The question of the second voice isn't answered in the provided excerpts.
Step-by-step explanation:
In the context of the excerpts provided, Dupin's conclusion seems to revolve around the cunning and psychological aspects of a crime, especially considering his observations on the robber's knowledge and the loser's awareness. The excerpts suggest the involvement of careful planning and the perpetrator having insight into the victim's mind. Dupin hints at the importance of the susceptibility of the document to be produced or destroyed, based on current court affairs, as an essential element of the mystery. As for the 'second voice' question, it is not readily answered by the provided text, which doesn't offer enough evidence regarding any voices heard.