We report on long-range duet interactions among twelve wild groups of lar gibbons (Hylobates lar) in Thailand. Statistical analysis demonstrates that groups were more likely to respond with an answering duet to a duet sung by a neighbouring group than to one sung by a non-neighbouring group in the population. A distinctive pattern of response among neighbours was to wait until a neighbouring group had finished its duet before immediately answering with a duet, resulting in avoidance of overlap between the two duets. Non-neighbours did not exhibit this pattern. The effect is shown to be due solely to neighbour status and not to the degree of mutual audibility of the duets. There was no evidence that, when duets overlapped, the first group to sing modified the length of its duet in response to the second duet, whether given by a neighbour or by a non-neighbour. In general, among those groups which responded to one another's duets, there were no identifiable leaders and followers: the order of duetting groups was random. We discuss why neighbours interact more by duet than do non-neighbours, and consider what may be the functions of avoiding overlap of duets.