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I made a test Product with one color and 4 sizes (39-40;   41-42;   43-44;   45-46)

I import via csv with this quantities (1 x 39-40; 5 x 41-42; 5 x 43-44; 1 x 45-46) once. I does it right.

I import the same again. In this case I suppose I´ll get (2 x 39-40; 10 x 41-42; 10 x 43-44; 2 x 45-46)

But no. I get (1 x 39-40; 5 x 41-42; 5 x 43-44; 1 x 45-46) and (1 x 39-40; 5 x 41-42; 5 x 43-44; 1 x 45-46)

This is a mess!!  How can update new arrivals (dozens or hundreds) if with just a single one this happens?
In the front office I get the Select size with just those 4 sizes, and the quantities applies only for that first import.

In backoffice I get eight combination lines instead 4

Is it a bug or I´m doing it wrong way?