No two electrons in an atom will have all the four quantum numbers same. This statement is known as

According, to Pauli's exclusion principle "no two electrons in an atom can have the same values of all the four quantum numbers."
In $1s^2$
for I electron $n=1,l=0,m=0,s=+\frac{1}{2}$
for II electron $n=1,l=0,m=0,s=-\frac{1}{2}$
It means if the values of n, l and m are same, then the value of spin quantum number must be different, i.e. +1/2 and -1/2.