Duplication of cards is possible ( and easy ), if a player has the context menu open for a card which is taken by one player that player can select the pick up card option and also take it creating a card in both players hands. This is happening with decks that have unlimited number of cards I have not tested this with decks with this option disabled. As a GM the ability to preview other players hands is inconsistent, some times I can some times i can't, reloading roll20 always fixes this however. I am less able to reproduce this bug though.