The study examines the interactive functions of gesture repetitions as a specific form of interpersonal synchronization in argumentative decision-making processes of peers (1-6 graders). Based on a collection of 13 instances, gesture repetitions are investigated in the process of collaboratively co-constructing arguments. Drawing on multimodal interaction analysis, a description is provided of how gestures are aligned and laminated with other multimodal resources, such as body posture and gaze, into multimodal repetitions during the co-construction of both arguments and counterarguments. The study illustrates the way in which multimodal repetitions serve to mark coherence between the participants' turns and contribute to the argument. As the multimodal resources are highly synchronized with each other, multimodal repetitions also make the collaboration of specific participants publicly visible to all interactants, thereby attaching more importance to the sharedness--and persuasiveness--of the constructed argument. The analysis therefore focuses on both depictive and pragmatic gestures that are conceived as parts of multimodal gestalts (Mondada, 2014), and describes different trajectories of co-constructions that entail interactively successful as well as unsuccessful examples.