Ok.. If I understand you correctly, the first bullet kills the zombie. Then the zombie starts playing "Dead" animation. If you shoot again, the second bullet should fly through the zombie (not collide with it). Is this correct?
You can disable collisions after the first bullet and disable Platform behavior for the zombie.
Or you can check if animation "Dead" is playing on bullet collision:
Bullet on collision with Zombie:
and Zombie animation "Dead" NOT playing (right-click and invert this condition)
...Zombie start "Dead" animation
...Zombie set collisions disabled