Tessa, Duncan's French girlfriend / fiance from the first season and first four episodes of the second, was not meant to be dropped. The actress was finding it difficult to shoot for six months away from her home (in the Netherlands; it was painfully obvious she didn't speak French natively when she pronounced the Seine river as "de SEEN") and asked to be released, so the writers killed her off. I agree that there wasn't that much to her character and her death really advanced Duncan's. There's only so many damsel in distress stories you can do with one person.
Amanda was a great character and perfect foil to Duncan. Even so, it was a mistake to give her her own series by fundamentally changing her character and trying to make her a heroine when she was, and could only ever be a thief in mind and occupation. It didn't help that she had no chemistry with the male lead (and, I'm told, the actors didn't get along at all).