Join the invoice lines for same items with same prices

Hello experts,

we want to join the invoice lines by the grouping rules and I'm not sure how it works...

Here a example:

Order

Item A 10 $     1 piece

Item B 20 $     1 piece

Item A 10 $     1 piece

____________

Sum              40 $

Actual invoice lines:

Item A 10 $   1 piece

Item B 20 $  1 piece

Item A 10 $  1 piece

____________

Sum             40 $

required invoice lines:

Item A 10 $   2 pieces

Item B 20 $ 1 piece

____________

Sum             40 $

Thanks in advanced!

Kindly regards!