In a Set repetitions are not allowed. means Set $\big \{ a,b \big \}$ is similar to Set $\big \{ b,a,b \big \}$, because Ordering and repetition doesn't matters in a Set.
But in a Multiset, Repetition matters. So, $\big \{ a,b \big \}$ is different from $\big \{ b,a,b \big \}$. Cardinality of Multiset is the total number of elements in a multiset, including repeated memberships. So, Cardinality of $\color{blue}{\big \{ M,I,S,S,I,S,S,I,P,P,I \big \} = 11}$
