Every time I buy Skype credit using iTunes, I get billed twice. For example, for £2.99 worth of credit, I get charged £ 5.98. Why?

You will need to try contacting iTunes Support. Either try the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via the Store > View Account menu option and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that in-app purchase and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, about 24 hours).

If the 'report a problem' link doesn't work (it's been taking some people to this site on a browser instead of showing a form in iTunes) then you can try contacting iTunes support via this page : http://www.apple.com/support/itunes/contact/- click on Contact iTunes Store Support on the right-hand side of the page.