How to identify pairings and repeats in population data

Try this:

=IFERROR(IFERROR(INDEX($D$1:$D1, MATCH(A2&"-"&C2, INDEX($A$1:$A1&"-"&$C$1:$C1, 0), 0)), INDEX($D$1:$D1, MATCH(C2&"-"&A2, INDEX($A$1:$A1&"-"&$C$1:$C1, 0), 0))), MAX($D$1:$D1)+1)