Suppose cells A1:A11 contain...
{"Field1"
;10
;10
;30
;10
;25
;10
;25
;30
;30
;15}
For Field2 (B2:B11) enter the formula, =1/COUNTIF($A$2:$A$11,A2), into cell B2 and copy down.
Create a PivotTable with Sum of Field2 in the DATA area. As you can see there are 4 unique Field1 values.