Avoiding Duplicate Content Penalty

how much rewriting is necessary?

[old_expat]

I have an info type web site that has been around for 6 years. It was primarily used for Topic "A". Recenty I started adding additional content on Topic "B", which is somewhat related. Site A is starting to earn a few $/day with adsense.

I am getting quite a lot of good quality content ready to add that should boost the exposure of Site A in SEs

Today I happened to check the availability of a non-hyphenated dot com. Gadzooks! It was available so I registered that sucker!

The only problem with old domain (Site A) is that it is a bit longer, hyphenated .. and not very intuitive,

The new domain (soon to be Site B) consists of 2 very good keywords that are spot on Topic "B" and the content I'm getting ready.

The content is original and I own the copyright. I prepared much of it myself; a published author friend is doing another series of articles (free); and I have contracted for an additional series of articles with a good freelancer.

So what I'm wondering .. how much rewriting do I need to do/have done to avoid any duplicate content filters/penalties.

I would really like to use the new (Topic B) content on Site A while Site B is escaping the sandbox .. and possibly thereafter.

[old_expat]

bump