Thanks for the pointers.
To add my music, I do it album by album - having been bitten by things going astray with a global import in the past. I do File-> Add Files to Library and then select all of the files in the album directory. Often, I have to re-group them but there is no box anywhere in this method that allows me to hide or allow duplicates on import.
I've checked the inbox and there is nothing in there.
In the Manage Duplicates Window, I have options to: "Hide Duplicates", "delete", "replace with reference to the kept file", and "save to playlist".
I've tried selecting all of the options in the "selected tags" but when I close the Manage Duplicates window, it just resets them to "artist" and "title".
I'm happy to accept that it's something I'm doing wrong but I can't see what that might be.