There are huge discussion what is better from a SEO perspective, having the category in the URL or not.
Magento creates a rewrite for each category the product is in. Turning on the
Canonical URL as @Dan suggested, avoids penalties for duplicate content.
The only "out of the box" way to have only one URL is, to turn category in URL off. (this is what I heard at the moment the recommended way, but I'm no SEO expert)
To have only one URL per product, you have to remove the category from the URL:
System > Config > Catalog > Catalog
> Search Engine Optimization > Use Categories Path for Product URLs: no
Answer on the comment
What I want, is the full categoriers path for every product, also if I click on recomended, you're telling me the complete opposite right now ;)
Yes, I did. To fix this I think you should still turn it off and change the indexer which writes the url rewrite, to use one (or the only) category of a product, so when a product url is requested from the url_rewrite, you get the one with the whole categories in it.