David,

When I'm looking for dupes I do

SELECT KeyName, COUNT(KeyName) AS NumDupes ;
FROM TableWithDupes INTO CURSOR MyDupes ;
GROUP BY KeyName ;
HAVING NumDupes > 1

To take advantage of Rushmore, index on KeyName

>I have a large table in which a small percentage of the records have identical values in a particular key field. What is the most efficient way to count the number of records having a particular value in the key field. If I'm only interested in a particular key value, searching by filtering seems like a pretty sexy way to do it, but do the realities of filter implementation indicate against it for large files?
>
>1) ordering off, count for keyname = value all
> (key is in active .cdx file)
>2) order by key, count for keyname = value all
>3) order by key, seek keyname = value, count for keyname rest
>4) ordering off, set filter to keyname = value then count all
>5) order by key, set filter to keyname = value then count all
>6) order by key, seek keyname, set filter to keyname = value
> then count all
>7) order by key, seek keyname, iterate a do loop to count
> while skipping
>8) something else I haven't thought of...
>9) Rushmore is so efficient that for a the trivial key expression
> given above, all of these are about the same.