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Why would you go for example.com/product/luxim-FZ200.html instead of example.com/luxim-FZ200.html ?

Duplicate content is not at problem - there will of course always be a default category.

What structure will recommend to the product pages?

Lets make an example with the keyword "Luxim FZ200"

With category in url:
www.myelectronicshop.com/digital-cameras/luxim-FZ200.html

With /product prefix:
www.myelectronicshop.com/product/luxim-FZ200.html

Without category in url:
www.myelectronicshop.com/luxim-FZ200.html

I have read in a blog post that Paddy Moogan recommend /lluxim-FZ200.html - i think i prefer this version too.

But I can see that many of the bigger ecommerce stores are using a /product prefix before the product name. What is the reason for this? and what is best practice?