Context: Double-counting in a literature review occurs when the same data, population, or evidence is erroneously counted multiple times during synthesis. Detecting and mitigating the threat of double-counting is particularly challenging in tertiary studies. Although this topic has received much attention in the health sciences, it seems to have been overlooked in software engineering. Objective: We describe issues with double-counting in tertiary studies, investigate the prevalence of the issue in software engineering, and propose ways to identify and address the issue. Method: We analyze 47 tertiary studies in software engineering to investigate in which ways they address double-counting and whether double-counting might be a threat to validity in them. Results: In 19 of the 47 tertiary studies, double-counting might bias their results. Of those 19 tertiary studies, only 5 consider double-counting a threat to their validity, and 7 suggest strategies to address the issue. Overall, only 9 of the 47 tertiary studies, acknowledge double-counting as a potential general threat to validity for tertiary studies. Conclusions: Double-counting is an overlooked issue in tertiary studies in software engineering, and existing design and evaluation guidelines do not address it sufficiently. Therefore, we propose recommendations that may help to identify and mitigate double-counting in tertiary studies. © 2023 The Author(s)