simplifying your original formula:

=IF(COUNTIFS(B:B, B2,C:C,C2)>1,IF(COUNTIFS(B:B,B2,C:C,C2)>1,IF(COUNTIFS($B$2:B2, B2, $C$2:C2,C2)=1,1,0),""), 1)
becomes:

=IF(COUNTIFS($B2:$B$106, B2,$C2:$C$106,C2)>1,0,1)