Social discrimination based on geographic variation in territorial signals is taxonomically widespread—most studies have found stronger reactions to local than to foreign signals. In birds with male-only song, this discrimination is thought to result in social exclusion and has been suggested as a behavioral barrier to interpopulation genetic exchange. However, little information exists on duetting species in this context, and nothing is known of how “mixed-dialect” pairs are perceived, despite their confirmed occurrence. We addressed these deficiencies using a duetting, endangered Passerine, the kōkako (Callaeas wilsoni). We used reciprocal stereo playback experiments between 2 fragmented populations to present duets from local and foreign dialects (Experiment 1). Additionally, we assessed responses to mixed-dialect pairs by synthesizing duets, a novel technique, to determine which sex contributes the most salient duet components (Experiment 2). Territorial pairs vocally responded to local duets with less delay, and produced more song phrases with a lower diversity, than they did in response to playback of foreign duets. Pairs responded to mixed-dialect duets with equal overall strength regardless of the sex of the local component. Responses to mixed and pure local duets were qualitatively similar. From an evolutionary perspective, this suggests that mixed-dialect pairs can successfully defend territories without social penalties, improving the likelihood of cross-dialect gene flow. These findings have particular importance for conservation efforts like translocation, which often involve individuals from multiple, culturally distinct populations.