I want to solve Problem 4.10 from Randomness by Salil Vadhan. https://people.seas.harvard.edu/~salil/cs225/spring15/PS3.pdf
Consider a bipartite expander $G$ with left degree $D$ so that every subset $S$ of the left vertices with at most $K$ vertices has at least $(1-\epsilon)D|S|$ neighbors. Then $G$ also has the property that it has $(1-2\epsilon)D|S|$ unique neighbors. Unique meaning that it has exactly one corresponding vertex from $S$.
I recognize that the new expansion factor is $(1-2\epsilon)D = 2\cdot(1-\epsilon)D -D$