Not the most "proof providing picture" but when hunting in Eos Tower, I received Block Golem card from a drop. When I got a King Block Golem card to drop, it replaced the effect of my Block Golem card. IDK if this is intentional, but it happened.
Monster card effects work like potions, if there's an active skill/potion that gives a certain stat, and the new monster card effect has the same effect in that stat, it overwrites it. So basically, it cannot stack. Also, a skill or pot that had all its effects overwritten will be removed from the buff list, cause it literally gives you nothing from that point on. That means, if, lets say, both these cards give some amount of accuracy, if one is active, and you pick the other one, the first one will get overwritten and disappear because its the only stat it gives. (tl dr, not a bug )
