Yes, you're correct that the union would return 20, not 10, in the case you
described. That's why we needed more information from you!

Okay, so you've got the following:

Id Field1 Field2
1 5 10
2 10 10
3 10 15
4 15 5

and you want to return

FieldValue Count
5   2
10   3
15  2

Correct?

Create a query that returns all of the unique values in the table and name
it qryUniqueValues:

SELECT Field1 AS LookupField
FROM MyTable
UNION
SELECT Field2 AS LookupField
FROM MyTable

Create a 2nd query that joins qryUniqueValues to your table and name it
qryIntermediate

SELECT MyTable.Id, MyTable.Field1, MyTable.Field2,
qryNewsgroupQuestionUniqueValues.LookupField
FROM MyTable
INNER JOIN qryValues
ON (MyTable.Field1 = qryUniqueValues.LookupField)
OR (MyTable.Field2 = qryUniqueValues.LookupField)

Create a 3rd query base that figures out the counts using qryIntermediate:

SELECT LookupField, Count(Id) As Total
FROM qryIntermediate
GROUP BY LookupField

Now, assuming you're using Access 2000 or newer, it's possible to combine
all of that into a single query, but I wanted to explain the steps:

SELECT MyData.LookupField, Count(MyData.Id) AS Total
FROM [SELECT UV.LookupField, MyTable.Id
FROM MyTable
INNER JOIN
(SELECT Field1 AS LookupField FROM MyTable UNION SELECT Field2 FROM MyTable)
AS UV
ON (MyTable.Field1 = UV.LookupField) OR (MyTable.Field2 = UV.LookupField)].
AS MyData
GROUP BY MyData.LookupField;