Daggerwings are similar in wing shape to Swordtails and Swallowtails (Papilionidae), but Papilionidae have 6 legs whereas Marpesia and all other Nymphalidae genera have only 4 legs. 
Another feature to look for is the antennae.
In all Papilionidae species these are recurved at the tip, but in Marpesia they are straight.

Marpesia berania looks similar to Marpesia harmonia.
In males (like on the photos), Marpesia berania has a black and white spotted patern on the underwings between the "tails".
This are absent in Marpesia harmonia.
In addition, the underwings of Marpesia harmonia are much more white and the top of its forewings are pointed and not round as in Marpesia berania.

Reserva Ecologica Taricaya, Puerto Maldonado, Peru, 16 June 2009