I'm not too familiar with Form 2555, but I'm not aware of a need to indicate dual citizenship. Maybe I'm wrong?
It is likely our expert in foreign matters will stop by in a while, but in the meantime maybe you can clarify why there is a need to report dual citizenship. Are you just trying to claim the Bona Fide Residency?
There is no input for dual citizenship. You will just use USA as the default.
In the event you are trying to claim treaty benefits, you will need to take a consistent position. US citizens are subject to the Saving Clause. Your client can't be tax residents of both the US and another state; Article 4 has a pecking order to resolve that.
Still an AllStar