Why do the test twice?
Because if a pet had been exposed prior to the first test there is a possibility that it would not show up for at least 4 weeks. And since Annie was in an outdoor environment it was highly possible for her to have been exposed AND to test negative the first time.
In reality ALL pets should be tested twice, at 4 week intervals, if they originally came from an environment where exposure was likely. (assuming the first test was done right after leaving the this environment)