Hi Prashasti!

It is true that polyspermy is prevented by the hardening of the zona pellucida. That means, only one sperm fertilises with the egg. 

Twins are formed under the following two conditions:

1. When the zygote undergoing cleavage splits into two embryo, where each embryo gives rise to a baby simultaneously, twins are produced. Such twins are called identical twins.

2. Under rare conditions, when each of the two ovaries releases an egg and the sperms fertilise each of these two eggs, another type twins are produced. Such twins are formed by simultaneous fertilisation of two different eggs by two different sperms and are called fraternal twins.

Thanks