What Bogi said, if they are unaligned to a God then it's just a generic tail which allows them an extra standard attack. Consider it like an oversized dino or reptile tail or something. Can't think of many other tail like things which would have enough clout otherwise! They should only get the other versions with their bonuses if they are aligned to a god. However what's done is done and while the rules say one thing, you as GM can say another. Maybe Khorne has his eye on them despite them not being fully aligned!
So to the question at hand, generally for a scorpion like tail, from watching how they attack it tends to come up and over their torso and I dont believe they can move it side to side that much. So it would need to come up and over. Whether they need to duck probably depends on the length of the appendage, what you're hitting and where as well as the size of the character. If you have given them a long enough tail it may be able to reach up their back and over the head to strike the prey. I would probably argue that based on the description of the tails use (an extra attack) that a duck action would require them to perhaps sacrifice some action in order to use it thereby mostly negating the extra limb so probably to answer your question:
"No, I would not say they needed to duck based on the description of the gift".