Formula:
=IF(C18=0,"N/A",INDEX('List A'!$A$7:$A$26,MATCH(SMALL(INDEX((('List A'!B$7:B$26<>$B10)*1000)+ROW('List A'!B$7:B$26),0),1),INDEX(ROW('List A'!B$7:B$26),0),0))) &
IF(C18>1,", " & INDEX('List A'!$A$7:$A$26,MATCH(SMALL(INDEX((('List A'!B$7:B$26<>$B10)*1000)+ROW('List A'!B$7:B$26),0),2),INDEX(ROW('List A'!B$7:B$26),0),0)),"") &
IF(C18>2,", " & INDEX('List A'!$A$7:$A$26,MATCH(SMALL(INDEX((('List A'!B$7:B$26<>$B10)*1000)+ROW('List A'!B$7:B$26),0),3),INDEX(ROW('List A'!B$7:B$26),0),0)),"") &
IF(C18>3,", " & INDEX('List A'!$A$7:$A$26,MATCH(SMALL(INDEX((('List A'!B$7:B$26<>$B10)*1000)+ROW('List A'!B$7:B$26),0),4),INDEX(ROW('List A'!B$7:B$26),0),0)),"") &
IF(C18>4,", " & INDEX('List A'!$A$7:$A$26,MATCH(SMALL(INDEX((('List A'!B$7:B$26<>$B10)*1000)+ROW('List A'!B$7:B$26),0),5),INDEX(ROW('List A'!B$7:B$26),0),0)),"") &
IF(C18>5,"...","")
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