Data Output
abc Dup1
abc Dup1
123 Dup2
123 Dup2
456 Non Dup
678 Non Dup
890 Non Dup
886 Non Dup
123 Dup2

Apply below formula in B column which is output column with CSE

=IF(COUNTIF($A$2:$A$10,A2)>1,IF(COUNTIF($A$2:A2,A2 )=1,"Dup"&MAX(IFERROR(SUBSTITUTE($B$1:B1,"Dup","") *1,0))+1,"Dup"&MAX(IFERROR(--SUBSTITUTE("Dup"&$B$1:B1,"Dup",""),0))),"Non Dup")

apply formula with Ctrl+Shift+Enter in b2 and drag down