This question assumes that this citizen has another passport (eg from dual citizenship). I thought to ask because identical questions for different countries yielded disparate answers. Both of the following quotes can be found in the penultimate para in each answer. If existent, please cite official sources.
UK: The UK does NOT have such a requirement. ie, you can enter the country using either your US or UK passports
USA: Also be aware that as a US citizen you MUST enter the US as a citizen. ...