When I plug in my Arduino Pro Micro, a serial port gets created in Windows, and the port disappears when I unplug it (expected), as seen in Window device Manager. The Arduino IDE on the other hand seems to have trouble keeping track of the comings and goings of the USB serial port.
From a fresh Windows boot, I can start everything up and load a sketch no problem. The port setting on the IDE is COM9, which matches what Windows device manager says. Some times I can even reload the board a couple of times successfully. But after one or 2 times, when I try to load it says "Couldn't find a board on the selected port." When I look at the port setting in the IDE, there are 2 or more entries that say COM9, and they're all checked. Windows says only 1 COM9 exists. Restarting the IDE makes no difference. I have to reboot the whole PC to get it to work again.
The latter is fixed in Arduino IDE 1.8.10. They are both fixed in the beta build of the Arduino IDE. Since the beta build is less likely to be stable and the former bug is merely cosmetic, I recommend that you update to Arduino IDE 1.8.10 and ignore the harmless duplicate entries in the Tools > Port menu.
Ok, thanks for the pointer to the bug report. I seems now the only way to get the production install package for Windows is through the Windows Store, and the version there seems to be still 1.8.9 (it says already installed when I try to download it). My running version is 1.8.9 (Windows store(22.214.171.124).
I downloaded the beta, which seems to be 1.9.0 build 133 at the moment. Only available as a zip file it seems, with no installer that I could find. The installation instructions assume an installer, so not sure how to proceed from here, except that the bug report says the upload actually does succeed, it just fails afterward, so maybe I’ll just keep using 1.8.9 for now.