If the two legal guardians have agreed to be contacted by the institution, will all correspondence relating to their child be sent in duplicate?

Yes, they will each receive a letter. To avoid sending two letters to guardians who live at the same address, go to display Resources > Guardians > Information form, launch the command Edit > Reset all the recipients > Report card and mail and validate: a single letter will be sent when the parents live at the same address (both parents will be recipients of the mail), two letters will be sent when they each live at a different address.