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%7E is the ascii code for ~. I'm not certain whats happening, but sometimes when copy and pasting rich text (like from word), the browser will see the ascii code and not the actual ~ character. This makes internal linking a bit awkward and can result in duplicate content, although I'm not sure if search engines would see this example as a problem. I've noticed that you do not have rel="canonical" tags set up, it's worth putting them in just to be on the safe side. I would also have a word with your developers as the urls are very long and messy. Also, try to avoid using upper case and avoid using non-alpha numerical characters (such as ~) except for hyphens. Something like http://www.arkwildlife.co.uk/straight-foods/sunflower-seeds/premium-sunflower-hearts would look a lot better and avoid problems like this occurring in the future. The only problem is that changing the url can temporarily affect search rankings and you must be sure to set up 301 redirects properly.