Special Calculation in Measure

Hello,

I have a table like the one below.

I calculate some numbers by filtering with the measure I write here.

As an example, I take with the following measure how many UD code columns are empty.

PF Order Sayısı =
CALCULATE(COUNTA('Sheet1'[Order]), 'Sheet1'[UD code] IN { "" })

It gives me 19, and this number is correct.

But there are 2 of them in the same order number in my table. 

I want it to count 1 time with the same order numbers in my measure. So my total result should be 17. How can I do that?

Regards,

Koray