I need some basic editing on a Google spreadsheet

Hi, I have a Google spreadsheet with 4691 rows in, each containing a URL. Most of these URLS are duplicate in the sense that they contain the root domain e.g. www.example.com/abc and www.example.com/xyz but we only want one example of the URL.

We have highlighted in column B where duplicates exist in red to make this easier to see. We need only one example of the URL to remain.

http://www.screencast.com/t/CjNDe6VL4lYp

Please do not delete all the red columns, we need one version of the domain to remain.