Hi and welcome to the forum

Thanks for the title change. If I understand you correctly, based on your sample data....
A
B
C
1
 PO # PO Correct?
2
300415
TRUE
3
4
300697
TRUE
5
300697
TRUE
6
7
301220
TRUE
FALSE
8
301220
FALSE
FALSE
9
301220
TRUE
FALSE
10
301220
TRUE
FALSE
11
301220
TRUE
FALSE


C2=IF(COUNTIFS($A$2:$A$11,A2,$B$2:$B$11,FALSE)>0,FALSE,"") copied down