Try something like:

 awk 'NR%2{if(p!=$2){v=$1; p=$2}$1=v}1' FS=, OFS=, file

This tests if `$2` has changed and substitutes `$1` on every odd line with the same value for every `$2`

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Added an explanation..
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 awk '
 NR%2 { # On every line where NR (recordnr) mod 2 equals 1 (odd line)
 if(p!=$2) { # If the variable p (previous) is different from the 2nd field then
 v=$1 # The new value v should become $1 (1st field)
 p=$2 # The new variable p becomes the current field 2
 } #
 $1=v # On every odd line field 1 becomes the previously set value
 }
 1 # 1 means true. The default action is to print the record (line)
 ' FS=, OFS=, file # Set the input and output field separators to a comma.