The probability of choosing a 6 without replacement is different from that of choosing a 6 with replacement.
First draw: P(6) = 4/52 = 1/4 (there are 4 6's in the full deck)
Second draw: P(6|6 was previously chosen and not replaced) = 4/51 (there are 3-6's left among 51 cards)
P(a 6 is chosen, NOT replaced, and another 6 is chosen) =
(1/4)(3/51) = 1/68