This
I support Tenhou, i like a team to have differet units, whenever i need 2 of a single class i go with either different rarity or different unit of the same rarity (except when i need 2 calliopes because we have no other silver witch... i can handle going with chloris but for maps like War of magic where you need mincost silver witches... you get the point)
Even if could ever manage to roll a second copy of Sybilla or Nanaly i would combine thim w/o a question (Nanaly after CC for that skil up of course)
But that's my opinion, and the decision is yours in the end