If I can throw in some of my experience here. So far you are talking about sites, but Google can do a LOT more harm than just sites. If both sites have duplicate content, and both sites have deep links to different pages, i have seen on numerous occasions half of the pages from BOTH sites dropped (so that if you put both sites pages together you get one site (if you see what i mean).
The result of this is the the whole flow of link juice between BOTH sites internal linking is slaughtered and they drop out. Not because of a penalty as such, but because pages were removed and Google left in the pages that it considered the most relevant (killing the structural flow of both sites along the way.
Who said
SEO is simple LOL
Oh & I have to agree with Webnauts, if this is your level of understanding of
SEo you shouldn't be charging for it. A little knowledge is a dangerous thing (as they say).
Once you know
SEO/Internet marketing, you will take ABSOLUTELY no crap from your clients. They are PAYING YOU to do what is best for them, so stand up to them and tell them they are damaging their business.
If they can't understand what you are saying or refuse to accept it, then walk away from them and inform them that you are not prepared to ruin your own good name in allowing a client site to get banned.