Sandboxed Domain - Options?

thinking of starting on a new domain

[PhilDC]

Hi,
My domain has been either in the sandbox or penalised for some time now. I have cleaned up my act but it is becoming clear that it's unlikely to come back into the serps as it was previously.

I am considering moving to a totally new domain and my question is really how best to do this without incurring duplicate content penalties etc.

Ideally, I would like to have both sites running at the same time and am hoping that the new domain would not get the duplicate penalty as the old domain is 'sandboxed' anyway.

Are there any other options available?

Many thanks
Phil