I have a sitecore multisite setup.
i\'m currently struggling with the \"duplicate content syndrome\" were google bots indexes my sites and is able to access the content
The way that Sitecore resolves items makes it possible to access pages in multiple sites and with multiple domains.
If you have the following structure:
-sitecore
--content
---site1
----site1page1
---site2
----site2page1
And site1 has the domain site1.com and site2 has site2.com, you can always address an item with its full path. So for instance you can access site2's pages on site1 like this:
site1.com/sitecore/content/site2/site2page1.aspx
There are multiple ways of handling this in regards to SEO, but the simplest is using canonical links in the meta data, so that Google doesn't consider this as duplicate content. You can then add logic to render a meta tag with the url you want on all pages.
If you don't want to allow pages from one site to be shown on another site, you should create different domains for each site, and then use Sitecore security to disallow read access from one site to another. For instance you could create site1 as a domain and then restrict read access on site2 items in that domain.