I enabled One Instance mode through "Options" -> "Allow Only One Instance" in Process Explorer (I'm using the latest version v16.32). I also enabled the "Replace Task Manager" so the "Ctrl + Shift + Esc" shortcut key will launch the process explorer. I'm using windows 7 64 bit.
The problem is sometimes when I press the "Ctrl + Shift + Esc", two process explorer instances will be launched instead.
It isn't straightforward to reproduce the issue, you have to press the "Ctrl+Shift+Esc" by holding it a bit and then release it. You might have to try this many times until you see two instances of process explorer being launched. But I found out by pressing "Ctrl+Shift+Esc" two times in short interval seems to have a bigger chance to produce the issue.
I had confirmed the keyboard isn't the issue as I had tested it with different keyboards which gave same results.