Follow set confusion about inclusion
If the grammar includes $M \to \alpha N \beta$ and $\beta$ is nullable then anything which could follow $M$ could also follow $N$. In other words,

$FOLLOW(M) \subset FOLLOW(N)$

So we need to add everything in $FOLLOW(M)$ to $FOLLOW(N)$. It does  not oblige us to alter $M$'s follow set.

If there is also $M \to \gamma K \theta$ with $\theta$ nullable, then similarly we have 

$FOLLOW(M) \subset FOLLOW(K)$ 

so we also have to add everything in $FOLLOW(M)$ to $FOLLOW(K)$. That implies that the FOLLOW sets for $N$ and $K$ are not disjoint, but it really says nothing interesting about $M$'s FOLLOW set.