welcome to the forum. i was headed towards xlnitwit's first solution too because of your subject header, but it seems like you don't want the blue highlighted because column C is not the same? and not because they are not consecutive? if so, select the range you want to apply to (say from A2:D16)
go to Home tab -> Conditional Formatting -> New Rule -> Use a formula to determine which cells to format -> Format values where this formula is true:
=COUNTIFS($C:$C,$C2,$D:$D,$D2)>1
format