Hi,

Formula =IF(COUNTIF(O$7:O8,O8)=1,MAX(N$7:N7)+1,"") is counting the
row index.

This formula is making a list of unique items which I am further using
in a data validation list.
=IF(MAX(NCOUNT)<ROW(1:1),"",VLOOKUP(ROW(1:1),NLIST ,2))

The list is like

550
551
552
553
554
C-09
C-10
555
556
557
C-11

I want to have this validation list in accending order like

C-09
C-10
C-11
550
551
552
553
554
555
556
557

Thx.