=IF(A2="",0,1)

=IF(A3="",B2,B2+IF(A3=A2,0,1))

=AND($A2<>"",MOD($B2,2)=1)

=AND($A2<>"",MOD($B2,2)=0)

=AND(D2<>"",MOD(ROUND(SUMPRODUCT(1/COUNTIF(D$2:D2,D$2:D2)),0),2)<>0)
