Basically the title. Would it be more efficient to buy duplicates and recall them, or to wait for the target dragon's orbs to appear in the shop?

Personally I would use any that show up in events or the orb shop. I would not waste money or gems buying duplicate eggs because you have to also use food to level them to decent levels to get the most orbs out of them.

Fyi I have not done either, i rank as they present themselves i do not buy to rank.....anything.