Formula to apply to filtered data

[AZN001]

I using this formula to copy duplicates of column S (starting at S10) over to U (starting at U10) - =IF(COUNTIF($S$10:S10,S10)=1,S10,""). However, I want it to only apply to the filtered data meaning if a duplicate was found on S11 with S13 and the duplicate was moved to S11, the next time I filter with S13 and down, I want the duplicate to show on S13 because S11 is no longer a part of my list.

 

[AZN001]

So if the list is

S11 - 1 U11-1
S12 - 2 U12-2
S13 - 1 U13-blank
S14 - 1 U14- blank

I want to be able to do this after it is filtered without S11
S13 - 1 U13-1
S14 - 2 U14-2
S15 - 1 U15-blank
S16 - 1 U16- blank

 

[AZN001]

Does anyone have any idea how to do this?