Yes; I've seen that in other shows now and then as well, like Law & Order in the U.S. I think sometimes in the longer-running shows an actor comes on in a small role, and they like working with him/her, and after a couple seasons (series) have gone by, they feel they can bring them back without confusing people too much if they clearly introduce them as a new character.

Yes, she did. I thought it was a bit odd, as the earlier character had quite a prominent role.