I recently bought some of the starter kit and I noticed that after redeeming the codes on Magic Arena store not all the cards avaiable on those decks were added to my collection, but only those that were necessary to complete the decks. I did not know if that was an error or if it is how it should work so I submitted a request in the WOTC support page and they confirmed me that in general deck codes only give you the card that are necessary to complete it, and no extra copy.
So as an example if I already have a copy of "Goddrick, Cloacked Reveler" and I redeem the code for "Day of the Dragons" deck, the one of the 2023 Starter kit that contains a single copy of "Goddrick, Cloacked Reveler", no more copy of that card is added to my collection and after the redemption I will still have a single copy of it. And just like that for any card already in my collection in a number of copies equal or larger than those present in the deck.

That seems reasonable but I suggest you to specify this particular rule somewhere (I could not find nothing offical about it anywhere and because of that I submitted the request), maybe on the physical product itself or on the MTG web page. Infact somebody is likely to buy that product also in willing to obtain those extra copies on MTGA, and finding out that it is not going to happen if you already have the necessary copies could be disappointing.

Thank you and sorry if that rule is already specified somewhere