Here is a typical popup on Ubuntu, when inserting a USB stick:

Unable to mount 32 GB Volume
Device /dev/sdb1 is already mounted at `/media/usb0'.

But the same occurs on other OS's, e.g. Raspbian Pixel and presumably all others?

Note: the USB stick is mounted successfully, and the content appears successfully off of http://box/usb (this is only a question of reducing the confusing clutter that we're currently placing in front of teachers -- would be tremendous to make this useless/erroneous popup go away, if not now, then in future!)