Thank you for your response. At first glance I thought it was repeating what I had already tried, that is "restoring defaults" and "deselecting Hide folder merge conflicts". Neither of these changed anything. However, when I created a tmp folder , copied the file into there, then recopied to a folder where it already existed I got my dialog window back.
Now I ask myself what is happening here. I can see if I try to copy a file into its original folder , I then get a copy file ( no dialog window ). I think my problem stemmed from copying files from my "recent files" list, which I get when I first open File Explorer.
So I believe that to copy files reliably i.e get a warning if file already exists, I need to copy from the original folder to a different destination folder.