On face of it arguments appear to be sound. Google obviously has several billion pages in it's database and could, it appears, easily determine if content is duplicated. It also seems, again on face of it, that it's reasonable to check for duplicate content, as this is "mark of a spammer" and not necessary on web with hyperlinking available. At least, this is common wisdom.

However, sometimes what seems reasonable and possible is not: not by a long shot.

Let's begin with technical side of things. You've got domain x and domain y with exactly same content. How on earth would Google be able to figure that out? Let's say Google had 3 billion pages in it's database. To compare every page to every page would be an enormous task - quadrillions of comparisons.

Now, if site x had page "page1" which linked to site y which also had "page1", then it would be possible for Google to determine duplicate content. Conceivably, it could check this out.

Not only is task enormous, but benefit is so tiny as to be insignificant. Duplicate content does not imply in any way shape or form spamming. In actual fact, a duplicate site is generally going to lower page rank of BOTH sites. Instead of having 100 links to one site, there will presumably be 50 links to one and 50 to another. This would tend (all things being equal) to lower page ranking of both sites. So Google gains nothing by this incredible expenditure of resources.

There are several reasons for duplicate content which have nothing to do with spamming. Sometimes content is actually duplicated, and sometimes it's just that there are several different domains (at least www and non-www versions) for same website

Mirroring a site for load balancing - This is very common. The purpose is to split up traffic between two copies of site.

Mirroring for region - Sometimes site mirroring is done simply to make it more efficient on internet backbone itself. You might put an identical copy of a site in Europe, for example, to reduce traffic across Atlantic, which should make it faster in European countries.

Viral marketing - It's extremely common to allow other sites to republish articles in return for a link.

Question: What if you have a lot of clients?

Chris: "Each client would have their own account. You would want to keep information separate and private. You can have multiple accounts under your main account. There is no limit to number of accounts or number of domains you monitor. Each account does require a separate license though."

Question: What do you mean by a separate license?

Chris: "Pricing for KeywordTool is based on a license fee and a click-tracking fee."

Question: So if you use tool for multiple domains/accounts, you'd have to purchase it separately for each domain/account?

Chris, "Yes, you can run all your accounts under your main account, but they are purchased separately."

Question: Is there a special setup fee for main account?

Chris: "No setup fee. The set up takes about 20 minutes and just requires adding a small JavaScript into head of your entry pages and a small piece of code into your conversion page. Then it goes live right away and starts gathering info."

Question: How much does service cost?

Chris: "The service usually runs from about $200/month per account and up depending on how many search engine related clicks you are getting per month. There are several examples of pricing at: http://keywordtool.net/pricing.asp."

Question: If I have 20 clients now, what would cost be per month?

Chris: "The first client is $150; 2-5 clients is $135 each up to 20. All clients after 20 are $60 each. It's progressive pricing."

Question: If I start with three clients, then add two more later on, does total subscription rate get adjusted to 5 clients, etc.?

Chris: "Yes."