How many final appearances has she had now?
She left at the end of S2.
She was back at the end of S4.
She got her goodbye at the end of Tennant's specials.
She's back again* in some form in the 50thFreema played "her own cousin" back in the series finale of S2, before she was cast as the companion for S3.
Karen played a Soothsayer in episode 2 of S4, "Fires of Pompeii" before being cast as the companion in S5.
Peter Capaldi played a Father in "Fires of Pompeii" before
being cast as the next Doctor, following Smith.Also, crossover to Torchwood, but Eve Myles played a character in Episode 3 of S1 (can't recall character name) before being cast in Torchwood.
And honestly, even prior to the reboot it was a fairly common occurrence for people to play different parts prior to being full-cast. The BBC tends to use a relatively small (seemingly) selection of actors when it comes to their shows. Once you've made it, you're in everything (not that I'm complaining. The talent is certainly worth it!).