I just found the Tome of the Inferno with one of my champions.
The tome unlocks Fire1 for the champion that reads it. Normally, this action is blocked from the Equipment screen. Hovering over the greyed out use button gives the tooltip "Unit already has this perk".
However, in this scenario I used the item on the loot pop-up screen. The result was that the unit, which already had Fire1, consumed the Tome but did not learn the perk.
The Use button was not disabled and I could click on it, even though it should be disabled since I already knew the trait.
