1024x768 at 100Hz, so hardly pushing it.

I am using the same captive cable on the monitor, have no choice. But when I use just the two cables back to back to connect one PC to the monitor there is no ghosting. The ghosting is caused either by the switch or the connections between the cables and the switch.

I'm not running some big test to compare different switch boxes. I have seen this ghosting with all the switch boxes I have used over the last 5 years or so.