IMPORTANCE Diprosopus is a medical condition that refers to full or partial craniofacial duplication. A particular subset of this condition, duplication of the mouth, is an exceedingly rare condition, with 7 reported cases in the medical literature. The embryogenesis and mechanism of disease are not well understood. The objective of this report was to describe a case of partial facial duplication with a discussion of the previous literature, leading to a proposed theory of embryogenesis for this rare anomaly. OBSERVATIONS We present a rare case of duplication of the mouth associated with an intraoral dysontogenic cyst, which presented with upper airway obstruction. The diagnostic and management strategies are discussed, as well as the histopathological features and theories of embryogenesis. CONCLUSIONS AND RELEVANCE On the basis of our findings, we propose the mechanism of origin for duplication of the mouth to be duplication of the first branchial arch. This case offers a deeper understanding of the mechanism of this disease than previously reported. Additional basic science and clinical research is needed to corroborate this theory.