I won't reply for the Duncan staff but I'll just state that IME*...
-many covers use a copper based plating (unplated covers being the best way to avoid that but a PITA to clean),
-all covers introduce a given amount of eddy currents and alter the resonant peak to some extent,
-covers used by big brands like Duncan or Even Gibson are magnetically transparent enough to make these Foucault currents acceptable or unnoticeable, depending on external factors. Not the same with covers on cheap Asian made PU's and mounted by Epiphone, Ibanez and so on (those being not transparent at all).
*Answer based on aural experience but also regular lab tests, BTW. ;-)
HTH.