I need a little help with a networking problem. I this has come up previously, I apologize for the new thread.

I just replaced my Airport Express with a new one. We run two Mac laptops. Other than the connection dropping out every once in a while, we have had no network problems in the past. With this new Airport, we now cannot get both laptops to connect at the same time. We keep getting messages about another device having the same IP address (and it always sets up the machine in use with this IP). I may have inadvertently configured something wrong in the Airport Utility when setting up the new router. I thought the IP addresses were assigned dynamically when DHCP is set, which it is). I have tried manually changing the IP address on one or other of the laptops, but this doesn't seem to work. Seems like this is unnecessary anyway. Any advice would be much appreciated.