Your PCs do not have the same IP Address internally, only as they are seen from the Internet. This is because you have private addresses provided internally by your router, which operates using NAT (Network Address Translation) or more accurately PAT (Port Address Translation) which shares a single IP Address on the Internet side of your router.
Internally your router, wireless or not, is providing a different IP Address to each PC or other network connected device like a PS3, network printer, etc. This is so it can identify that device when it sends something out to the Internet and then allow the response to come back to the internal network. It finds the actual physical device using its MAC (Media Access Control) address.
What happens is that your PC asks your router for an IP Address when it first connects, which the router supplies from a 'pool' or table of addresses that it keeps internally. It then remembers what IP Address it gave to which MAC address in that table, so it can look it up again in the future.
A
MAC Address is a hardware address embedded in the NIC (Network Interface Card) built into your PC or other device. This is assigned to the NIC by the manufacturer based partially on standards set by the IEEE. You might be able to figure out who manufactured the NIC containing a specific MAC address, since that's built into the first part of the MAC itself.
Since the MAC Address is physically located on the NIC card in the computer in question and that computer can be located anywhere that the WiFi signal can reach, it's obvious that this can't help find the actual PC. This is why everyone's telling you to secure your network, since prevention is the only way to protect yourself. You have no technical or legal right to claim anyone did anything wrong since you left your door unlocked (no security) and advertised that fact (broadcast SSID).
The moral question still exists, but is basically pointless since there's no way to police this after the fact. This, and the fact that you never stated that you'd turned on any specific protection is why you got the respone you did. Whenever you post in any open forum, you need to explain yourself fully or be prepared to get an invalid response. Think first, or many questions will get asked later.
Bitman
Also, not broadcasting your ssid will make your network invisible to everyone. 
, but I wanted SACD and PS2 and PS1 support. I'm not getting one in another year.