The first step in Remembrance duplication is locating a Walking Mausoleum, one of the massive buildings with stone legs that wander around swinging an enormous bell. Players will have to attack small patches of skulls on the Mausoleum, which are found either on the very top or on its legs.
Once attacked it will fall to its knees, then the player can enter the building and duplicate their Remembrances inside. Players can only duplicate one Remembrance per Mausoleum, but if they would like to duplicate another (or duplicate the same Remembrance more than once), they can always track down another Mausoleum elsewhere.
It appears that you can duplicate the same Remembrance multiple times at separate Mausoleums, though the rewards can only be acquired once. For example, by duplicating the "Remembrance of the Grafted," you can acquire both the Axe of Godrick and the Grafted Dragon, but the game won't allow you to get two Axes, nor two Grafted Dragons.