I have an idea for a mouth rinse I would like to patent. When performing a quick Google search, I found several patents that seem to have overlapping claims with each other. My patent would likely overlap as well. The question is why has the USPTO granted subsequent patents that seem to overlap with the prior ones. Here are two example patents:
Given @Epicentre's comment, I think the answer is that the two patents cover different inventions. Remember that the claimed invention includes every element in the claim. Although the car has been around for a century, I can still patent a design for a car, as long as it has some combination of elements that hasn't been done or described in the prior art before.* It appears that although both the patent and the pending application include the use of xylitol, they include other limitations that hadn't been done before, and weren't obvious to do.
*See section 102 for the technical details of what can disqualify a patent
Without delving into the patentability of overlapping compositions, the situation in this specific case is quite simple: US 2006/0286044 is not yet patented, it is the publication of a patent application. As such it has no other value than any other publication.
If you want to know whether the application is still pending and what the chances are of the claims that you are curious about being granted, you should check the status of the application in the USPTO PAIR system.
If you do that you will see that claim 1 has been amended in November 2012 and includes now the limitation that 'said xylitol is present in an amount from about 5% to about 20%, and said water-soluble calcium salt is present in an amount from about 0.01% to about 1% by weight'.