I’ve finally started collecting Legacy Items, and I collected A Dream of Red from both sources. It appears to be a "set this value to 1" sort of situation, so I effectively gained nothing from collecting it twice. It’s far from a crippling blow to my character, but it would be nice if it were a sellable item, or even just swapped it out for, say, some love stories if you try to collect it and already have one.
I assume gaining the other legacy items multiple times is similarly disappointing.