Countifs but ignoring duplicates in 1 column

I need to count the number of client reviews completed by each team for each month. However, a client may appear multiple times (it will be the same date each time, a client wont ever have 2 different dates) but I only want the client counted once.

Please can you advise on how to use countifs but not counting duplicates for the client ref number or if there's another method.

Thank you.

Also posted here: https://www.mrexcel.com/board/thread...-once.1260439/