If you assume the problem is correct, in other words one and only one of the four answers is correct, then you have to exclude the two identical answers (25%).

If you assume the problem is correct, in other words one and only one of the four answers is correct, then you have to exclude the two identical answers (25%). You are left with 50% and 60%. You pick one of them at random, and the chance to be correct is 50%. If you start with the assumption that the problem is wrong, then ... it's wrong.