Count Values in another sheet but avoid duplicates

Hi,

So in Sheet 1, I want to count the value 'EM' from Sheet 2 but avoid duplicate counting.

In Sheet 2 I have some client numbers and initials. I want to be able to count 'EM' but the value to be 1, as they have only raised 1 Invoice for Client 10 instead of it showing as 4.

Thanks for your help 🙂