Having more than one file open at once

My Pro program suddenly won’t allow me to have more than one file open at a time; keep getting message saying something about a file under my licence is already open and Sketchup will close??? Anyone have any ideas on this?

Sometimes if you are on a wireless network and a wired network at the same time, depending on how the LAN is set up, SU will see the second instance as a second machine. If you are wired into the network, try turning off the wireless receiver and see if that corrects the issue.