In many tropical bird species, partners combine their songs to form precise duets, of which the function is imperfectly understood. Duet structure and sex differences in vocal strategies may be important indicators of different selective pressures that have led to the evolution and maintenance of these complex acoustic displays. This study examines the singing behaviour of a population of the eastern whipbird, a bird that forms antiphonal duets initiated exclusively by the male. In all, 7% of duets recorded were between a paired female and a male other than her social partner. Males sang more often than females, their songs were longer and moved through a wider frequency range, and they had a larger song repertoire. Females sang two types of song: response songs, used primarily in a duet context, and structurally distinct solo songs, typically used during interactions with other females. Eastern whipbirds lacked unique song types among the repertoires of individual males and females. Males and females combined songs non-randomly to produce specific duets that were shared across the population. Results suggested that song and duet type matching might play an important role in intrasex interactions, such as defence of a territory, or a partner, from same-sex intruders.