Hi claire78,
The fact that someone is a dual-citizen is irrelevant to the current rules of the At-home GMAT. Currently, each person is allowed to take the At-home GMAT only ONCE. Since you are still you, "using" a different citizenship doesn't change the fact that you already took the At-home Exam. It's certainly possible that you might be able to 'trick' your way into taking the At-home Exam a second time by setting up a second account, but it would be fairly easy to connect the two profiles after the fact and then GMAC would probably VOID your Scores and put a 'red flag' in your profile (for purposely going against the rules of the Exam) that Schools who receive any of your Official Score Reports (including ones for Exams that you might take in the future) would then be able to see.
If you took the At-home Exam and ended up with a Score that you're not happy with, then the practical answer is to keep studying and retake the GMAT at a later date (either at a Testing Facility or again At-home IF the rules are changed to allow for that retesting option).
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How did you score on the GMAT?
2) How long have you studied? How many hours do you typically study each week?
3) What study materials have you used so far?
4) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
5) What is your overall goal score?
6) When are you planning to apply to Business School and what Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich