Domenic wrote:
[...]

D3, copied down:

=IF((A3<"")*(B3=C$2),IF(ISNA(MATCH(A3&B3,$A$1:A2& $B$1:B2,0)),LOOKUP(9.99
999999999999E+307,$D$2:D2)+1,""),"")

...confirmed with CONTROL+SHIFT+ENTER, not just ENTER.

[...]

Just ENTER will suffice.