It means exactly what is says. 2 or more devices have been assigned the same IP address.
It does not mean anyone is accessing your network.
It just means, for some reason an IP address assigned to one device got assigned to another one as well. This can happen if for instance your Mac was sleeping, and to the router it appeared as if the IP had been released, and so it assigned it to another device that connected to the network, then your Mac turned back on, but was still holding on to that IP address, and did not immediately get a new one. So it would show two devices using the same IP. This generally is resolved quickly when one of the devices requests a new IP address.
You can avoid this by having either manually assigned IPs, or device specific IPs at the router, so each device gets the same IP always. Then there's no chance of duplication.
But, yes, disconnecting all devices from the Wifi network, and power cycling the router should also get rid of it at least for a while.
Also, and Airport Express cannot be a modem. It lacks the hardware for that. Your Virgin Hub is still your modem as its what is connecting to your internet service, and I assume your Airport is just acting as a wireless hotspot. Assuming the Airport is not acting as a router and assigning IP addresses, then power cycling will get rid of the issue for a while.
Otherwise, if both the Virgin Hub and the Airport are assigning IP addresses at the same time, they eventually may try to assing the same address to different devices at the same time which causes the error. In which case you would need to make sure only one device is assigning IP addresses.
