Misleading port sometimes being offered when uploading. I'm using IDE 2.3.8.
It was on COM6 when programming with another Uno earlier. No other ports are connected. For uploads I instinctively first used the option for the current cable connection.
When you see what you describe in the IDE what does the OS show as available COM ports ?
I'll have to check the next time it happens.
Right now, restarting IDE 2.3.8 and repeating the exercise, Tools > Port is correctly showing only COM3 (and COM1). Device Mgr is showing:
Communications Port (COM1)
USB-SERIAL CH340 (COM3)
I had similar problems yesterday, but more serious, in that 'COM6 persisted after restarting IDE and PC, and other 'impossible' states.