Try to use formula
Enter in B2 and copy down
Formula: copy to clipboard
=IFERROR(INDEX(A$2:A$9,MATCH(0,INDEX(COUNTIF(B$1:B1,A$2:A$9),,),)),"")

v A B
1
2 SS17 SS17
3 SS17 SS18
4 SS17 SS19
5 SS18
6 SS18
7 SS18
8 SS19
9 SS19