Two domains on one Wordpress Installation

Question

I recently changed the base domain on my Wordpress website from http://www.fsdegrees.com to http://www.56degrees.co.uk. I would like to create various of 301 from old domain to new one, but for some reason I can still access pages using the old domain www.fsdegrees.com/blog I though it should come back with 404. I'm just afraid that Google will penalize me for double content.

Any idea why that's happening. Many thanks,