In all or most of my folders, my file names are duplicated but with a period-underscore "._" appearing before each duplicate. The duplicates don't open to anything except "This file type can't be viewed".
This may have come about when I briefly tried using a memory stick as a Dropbox folder. I moved my default folder back to my desktop when I saw that Dropbox does not recommend using a memory stick. But now I have these weird extra phantom files.