Sorry, my English is not very good. Doesn't the Word "another" imply that there was a previous Kill Roll?
Your English seems good to me. Overall, I think the Bastogne rules are some of the best I have ever seen for a wargame, but the particular sentence we are talking about could have been worded better.
Since there is a DG marker in the hex they are assuming that it was due to a previous artillery attack (This is not necessarily the case, but this is what they are assuming). Since there was a previous DG due to artillery, there must have also been a previous Kill Roll. That is why they say "another".
I would understand that an artillery unit can make only one Kill Roll if your sentence were "if an artillery unit scores a DG on a stack that already contains a DG marker it does NOT add another DG marker, but it does get to make the Kill Roll". It's the word another what confuses me.
I think I like your wording better.