Shell Programming and Scripting

awk '
BEGIN           {FS = OFS = "," }
$1 != last      { last = $1; x = int((1 + c++) / 2) }
$2 = x OFS $2
' Input.csv | awk -F, -v row=1 -v col=2 'FNR==1{} FNR==row{$col="Index"}1 ' OFS=,

GivenColumn,C2,C3
1,2,3
1,4,5
2,3,2
3,4,5
6,7,8

GivenColumn,Index(Generated COlumn),C2,C3
1,1,2,3
1,1,4,5
2,1,3,2
3,2,4,5
6,2,7,8

awk -v row=1 -v col=2 '
BEGIN           {FS = OFS = "," }
$1 != last      { last = $1; x = int((++c) / 2) }
FNR==row        {x="Index"}
$col = x OFS $col
' file
GivenColumn,Index,C2,C3
1,1,2,3
1,1,4,5
2,1,3,2
3,2,4,5
6,2,7,8