Ive recently wanted to do a LAN server with a couple of friends of mine. but we have a slight problem, you see: when we set up the computers, and turn on the server, we get the next computer to connect to the server, it gets an error saying, you've already logged in. and, the one computer that is in the server: here and there this is missing blocks. and one time, the bricks would re-appear after harvesting.

can you help with with this? (i used this forum to get as far as i did: http://www.worldofminecraft.com/node/5422)