Can you provide some sample data and I will have a look at this for you. Until then, I will make an assumption:
You could re-write / write a query which will work it out for you in advance.
-- Assuming your test data looks something like this???
DECLARE @TestData TABLE
INSERT INTO @TestData
AS ( SELECT Devicename ,
COUNT(Devicename) AS DeviceCount
GROUP BY Devicename
HAVING COUNT(Devicename) > 1
SELECT SUM(DeviceCount) AS TotalCountofDevicesWhereCountMoreThanOne
The above can then be used anywhere on your report.
Failing that, (The only way I can see) break down the problem by putting the returned data into a Tablix on the report and create an expression to return the count if it's greater than 1 otherwise return 0. The unfortunate part is that you cannot then use those values in an aggregation other that the header or footer of a report. In this case I have used the header and all looks good. (See image attached)