Duetting behaviour is a unique form of animal communication that occurs in many diverse taxa in the animal kingdom. Hypotheses for the function of duets can be grouped into two broad categories to explain why individuals coordinate with their partner’s songs to create duets: (1) duets are cooperative displays where pair members have a shared goal, such as joint resource defence, and (2) duets are conflicting displays where pair members have different goals, such as guarding against divorce or extrapair copulations. To distinguish between cooperative versus conflicting functions of vocal duets, we conducted a playback experiment to 40 breeding pairs of barred antshrikes, Thamnophilus doliatus. We broadcast five playback treatments to territorial pairs of birds: solos of males and females, duets created by males responding to female songs and by females responding to male songs, and a heterospecific control. We categorized subjects’ behaviour in terms of vocal and physical responses. Male vocal and physical responses were significantly higher towards conspecific stimuli than towards heterospecific stimuli. Males tended to show more vocal responsiveness towards duet stimuli, although this tendency was not significant. We observed a similar pattern for female vocal responses. However, for physical responses, females showed significantly more intense responses towards female solo stimuli than towards any other conspecific treatment, suggesting that female antshrikes perceive rival females as an especially intense threat. Our results provide some support for a joint resource defence function for duetting in this species, while the high intrasexual aggression observed between females may indicate a form of mate guarding.