Rank / Large function

Hi there, in column A i have numbers. Some of which are repeated.

I want to be able to rank teh numbers without the repeated ones counting twice.

For instance if my data in col a was

4
15
7
21
6
16
15
7
8
3
4

and I want to rank largest to smallest it currently shows

21
16
15
15
8 etc etc

How can i correct the formula so 15 only shows once?

Great thanks