DistinctCount overcounting when drill down by date

Hello

I have data that looks similar to the form below.

I am using a simple measure:

CountSN = DISTINCTCOUNT(Table[UnitSN])

I want to show how many SN's completed the questionaire over time, and I want to be able to drill down through the date range. As I drill down, the distinct count function starts counting SN's more than once. For example, when I look at CountSN by month i get this:

But when I look at it by day I get this:

Since there are only 3 unique SN's in the table, the latter is clearly not correct. How can I filter this so that it only counts each SN one time, regardless of the date range I choose to look at?

Thanks,

Jason