I love when sci fi settings/devices get organically woven into character arcs, so I'm a sucker for the "who's who" trope (heh). The situation with the doubles let us see Donna and the Doctor want to be vulnerable but also scared and angry. They do get that tender hug at the very end but even then that's sandwiched between the Doctor confusing the double with the real Donna and him not wanting to talk about "15 years."

I really wonder how the relationship will round out in just one special.