https://www.gregmat.com/quizzes/quiz/prepswift-set-unions-4hvy (Question 3)
Consider a set {1,1,2,2} and {1,1,3,3). Won’t the number of elements in union=3?
\{1,1,2,2\} = \{1,2\}, which has a cardinality (size) of 2 (and not 4). The same thing is true for your second chosen set too.