Usage and Grammar

Q. This sentence was in a script that I was given for a second round of edits: “The identifying information you provided is not valid for a new or an existing card member.” The an had been marked for deletion by the previous editor. I would have chosen to retain it because my understanding is that each adjective used in a series such as this should be accompanied by the article that would be correct if it were used alone. Which is correct?

A. You can write it either way. Although sticklers may believe that the an must be expressed, it will be easily supplied by readers. English writing is filled with implied constructions. Displaying every grammatical underpinning would make writing wooden and verbose.