Thanks for the suggestion. But it doesn't duplicate my situation, where field Name has label Title -- which I believe is the problem (see below).
I just used the Smartlist wizard (which I presume means, select from the dialog lists rather than type in the rule formula?)...
I picked "Only duplicates of" then selected Artist, Title. (Name wasn't an available field)
The resulting list has more than 48 thousand files -- and they are clearly not dupes.
The resulting rule is [Media Type]=[Audio] ~dup=[Artist],[Title]
I believe the problem is MC gets confused by a Label applied to a library field. Years ago I set MC to display Title for library field Name (an ambiguous term that confused my users about what to put in the field). But in some dialogs Name is shown instead.
Thinking this a source of trouble, I switched the library field label back to Name. After restarting MC, ~dup works! The smartlist now has 8813 files. I guess I'll live with Name instead of Title.<g>
But all is not perfect.... I noticed that the smartlist still showed Title as the view column heading, and listed the titles/names of all the music files. As an experiment, I went into customize view and added column Name (from the field list) to see if it worked and had the same data as the Title column. It is now listed in Columns to Show, but it doesn't appear in the view. If I right-click the view and in the Columns list select Name, nothing happens. Does MC prevent showing the same data column twice?
Worse: If I unselect Title, it totally disappears, even if I then select Name. Remember, I restarted MC after removing the Title label -- but I can't get the Name column to show. (Yet using in in the ~dup formula is necessary to make that rule work so it's obviously an active field.)
And, in the view show columns selection list (right-click view column header), after I unclick Title and click Name, the Title column disappears, but the Name column does not appear -- there's simply no name/title in the view. Worse -- I can't get it back. Title is no longer listed as a column to show, but selecting Name doesn't show anything. The view seems to be trashed. Fortunately, I have a master view design that I can reapply. It contains column Title so it reappears.
Surely, something is amiss, but what?
SCARY THOUGHT: Do I have a problem after years of putting data in the field labeled Title that I presumed (and Library Manager showed) is actually field Name?
This naming confusion is similar to some confusing experiences I've had where smartlist rules sometimes need the fieldname, sometimes the field label, and in some rules MC fights this by accepting only one while actually needing the other, or MC requires label in some parts of a rule but fieldname in others. And the wizard contributes because it shows, say, label when it really requires fieldname for a particular rule parameter.
Minor note that stumped me for 30 seconds: I deleted the label of fieldname Name hoping this would revert its behavior to the default. But in MC I kept seeing Title, not Name, in smartlist wizard. So I closed and reopened MC but still saw Title. Then I realized that in the background, MC Server was running -- not affected by closing MC. So I closed server too, then restarted MC and now I see Name instead of title. Nothing wrong, just a small distraction from the process of doing a clean MC restart.