I recently upgraded to calibre 3.11, and also got the Win 10 Creator's Update. I have Calibre on my taskbar, but now when I start it a second copy of the icon appears on the taskbar for the running process. This is what it looks like (hovering the mouse over the right-hand icon of the two):
I used to have only the one icon, underlined if running like the right-hand one of the pair, not underlined like the left one if not running. Clicking on the left icon tries to start a second Calibre instance, and after a few seconds takes me to the instance which is already running.
This doesn't happen with any of my other quick-launch taskbar programs. Is this a Calibre issue, or a Windows issue?