The original text doesn't use any articles. It uses the same word twice. Yes, it seems that "was with God" and "was God" are logically incompatible but in the context it makes sense:
Thomas answered him, “My Lord and my God!” (John 20:28)
No one has seen God at any time; God the only Son, who is in the arms of the Father, He has explained Him. (John 1:18)
I and the Father are one. (John 10:30)
Philip *said to Him, “Lord, show us the Father, and it is enough for us.” Jesus *said to him, “Have I been with you for so long a time, and yet you have not come to know Me, Philip? The one who has seen Me has seen the Father; how can you say, ‘Show us the Father’? Do you not believe that I am in the Father, and the Father is in Me? The words that I say to you I do not speak on My own, but the Father, as He remains in Me, does His works. Believe Me that I am in the Father and the Father is in Me; otherwise believe because of the works themselves. (John 14)
What would "a god" mean? Where else is "a god" in GofJohn? There is only one God mentioned.