Why can't you use if statements? Do you have to use VBA?
=IF(COUNTIF($A$1:A1,A1)=1,A1,"")&IF(COUNTIF($A$1:A2,A2)=1,","&A2,"")&IF(COUNTIF($A$1:A3,A3)=1,","&A3,"")&IF(COUNTIF($A$1:A4,A4)=1,","&A4, "")&IF(COUNTIF($A$5:A5,A5)=1,","&A5,"")&IF(COUNTIF($A$1:A6,A6)=1,","&A6,"")