??? "another device on the network is using your computer's ip address"

Most likely, not.

Almost definitely, the network is using DHCP - that's a system where the network router automatically assigns an IP address to each device on the network. Say you go out for a while. It's a hostel, so you undoubtedly took your device with you. While you were gone "your" IP address was not in use, so it was reassigned to another device. When you returned, your device tried to reconnect using the IP address it had previously been assigned, and guess what? You'll get exactly that message.

Sign out of the network, and sign back in. You'll get a new IP address, problem solved.