While playing Card-Jitsu Fire (to help penguins who are non-members), I noticed the same black card come into play. The first time, I used the black card - however, I couldn't tell what number it was. Then I got another one, this time with a different element, but the same card! I was baffled, and it wouldn't let me use the card! Over time, I had 5 black cards which I couldn't use.