Would you ever have an AoH like the following, and if so, what would be the right thing to do with it?

{
 id_a => 1
 id_b => 2
 value_x => 20
 value_y => 30
}
{
 id_a => 3
 id_b => 4
 value_x => 40
 value_y => 50
}
{
 id_a => 1
 id_b => 4
 value_x => 100
 value_y => 200
}
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Would you want "id_a==1" to come out with 120, and "id_b==4" to come out with 250? Or do you need to keep track if of distinct "id_a/b" tuples?