Rigidbody is Pushing Another Rigidbody : Unity

Hello Everyone,

I have one issue in my game.

I am having zombie gameobject with rigidbody but when they come in contact , at that time they force each other.

Zombie game object having box collider and rigidbody component.

Take a look at my scene :

Here they does not move ahead if they comes in contact with rigidbody and forces eachother in opposite direction.

They are having same Mass = 1.2

When zombie collides with fence i am setting Rigidbody constraints to "Freeze All "

So what should i do? If you guys dnt understand my question then feel free to ask me.

Thanks.