This is a temporary glitch, IOW, when I reload the page, the cross displayed messages disappear and only the correct messages are shown.
This happens very often, every day. I will click on a listing that I have sold and messages for two listings will be displayed.
The first time this happened, I freaked out, because I thought I had made a mistake and given the wrong item to a buyer, so I can imagine how this might cause anxiety for new sellers here when they run into it.