What does it mean when someone is using your IP address and what should i do...
It means that another device on your network has been assigned the ip address you are trying to use on your computer. Make sure all your connected devices are configured to use dhcp not with static ip addresses. Make sure that your network is secure so that others in the area can't connect to your network using a static ip address.
Expanding on Bob's explanation, be sure you haven't set a manual IP address, thus being the culprit for this problem. In System Preferences / Network / Advanced... / TCP/IP section, Configure IPv4 should be set to Using DHCP. If that is not the case, then some network miscreant has purloined the IP address your Mac was using.
First try clicking on the Renew DHCP Lease and see if the DHCP server figures it out and assigns a different address. Else, try disconnecting from the network (if on Wi-Fi, turn Wi-Fi off in the previous window, if cabled disconnect the patchcord), wait a little while and reconnect. Then hopefully the DHCP server will realize the address was purloined and assign a different one.
what does it mean when your computer says that someone is using your IP address
