I'm Doug, and I'm very sorry to hear of your situation. My goal is to provide you with excellent service today. Please don't confuse excellent service with a satisfactory outcome based on your wants and desires. I can only explain the law to you---not guarantee that you will be happy with the outcome---any more than a cancer surgeon can guarantee a cure.
Based on your statement of the facts, it appears clear that you did violate the one-house per copy of plans you paid for and which was a part of the sale agreement of the plans to begin with---the license you were granted when you bought the plans and you can be sued under 2 separate theories of law. The first, which they are alleging is copyright violation, they have only 3 years from the date that they first learn of your violation to file suit. The second possibility would be a suit for breach of contract, and under NC law, the statute of limitations for breach of a written contract is 3 years from the date of the breach---which would have been the time that you began construction of the second home with the same plans.
And it is true, that in a copyright action, the copyright holder may sue to recover the profits that you gained from the violation of that copyright.
I strongly suggest that you consider retaining a local copyright litigation attorney to assist in the preparation of your defense in the event that you are sued.
You may reply back to me using the Reply link and I will be happy to continue to assist you until I am able to address your concerns, to your satisfaction.
I hope that I have been able to fully answer your question. As I am not an employee of JustAnswer, please be so kind as to rate my service to you. That is the only way I am compensated for assisting you. Thank you in advance.
I wish you and yours the best in 2016,