In C1 Cell

xxx

In C2 Cell - Array Formula - Requires CTRL+SHIFT+ENTER

=IFERROR(INDEX($A$1:$A$25,MATCH(0,COUNTIF($C$1:C1,$A$1:$A$25),0)),"")

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Array formulas need to be entered using the key combination of CTRL,SHIFT,ENTER (not just ENTER). Hold down both the CTRL key and the SHIFT key then hit ENTER.