The correct command line tool to quickly check if two devices on the network have the same IP address is arp -a. By issuing this command, the technician can view the Address Resolution Protocol (ARP) table, which shows the IP addresses and their associated MAC addresses. If there is an IP address conflict, there would be multiple MAC addresses listed for the same IP address.

The ping command would not be the most efficient tool to diagnose an IP conflict because it can only confirm that a specific IP address is reachable, not whether multiple devices are using it. ipconfig (or ifconfig on Linux) displays the IP configuration of the host on which the command is run, but it will not show if another device on the network has the same IP. traceroute (or tracert on Windows) is used to identify the path taken by packets to reach a destination and would not reveal IP address conflicts.