Hi mark,
try this =SUM(1/COUNTIF($A$1:$A$100,$A$1:$A$100)) as array.
Adjust the range to your need and be aware this will only work if there are no blanks in your range.
An non array alternative could be: =SUMPRODUCT(($A$1:$A$100<>"")/COUNTIF($A$1:$A$100,$A$1:$A$100&""))
Hope this helps