Or this CSE array (Confirm with Ctrl+Shift+Enter, not just Enter)
Formula:
=IF(ROWS($2:2)>COUNT(INDEX(IF(COUNTIF($B$2:$B$20,$A$2:$A$20)=1,1,""),,1)),"",
INDEX(A:A,SMALL(INDEX(IF(COUNTIF($B$2:$B$20,$A$2:$A$20)=1,ROW($A$2:$A$20),""),,),ROWS($2:2))))
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