Keep in mind, consubstantialis is itself a translation of the Greek term homoousios, meaning “same essence.” This is of extreme importance to our faith and the doctrine of the Trinity, as it arises out of the Nicene Council, which was a response to one of the great heresies of the fourth century, Arianism. The Arians promoted the idea that the persons of the Trinity were homoiousios, similar substance. This is, incidentally, where we get the expression “it doesn’t make an iota of difference.” The only difference between Nicene Christianity and Arian Heresy is one iota, but what a world of difference it is!
Constantine’s son the Emperor Constantius was an Arian. Some of the most important bishops of the time were Arians. The controversy almost went the other direction, but under the wisdom of the Council of Nicea came our Creed. The translation “one in being with the Father” is simply not accurate, and does not reflect the true reasoning behind the Creed. Indeed, “one in being” seems pretty vague if you ask me, as an Arian would probably agree that the Father and Son were one in being, but differed in substance. The significant distinction contained in our Creed is that they are of the same substance, homoousios, consubstantial.
As to the grammatical issue, “consubstantial with the Father” is not really redundant. “Consubstantial” means “of the same substance” according to Webster, so it doesn’t really seem redundant in English to add the preposition. Moreover, the Latin phrase is “consubstantialem Patri.”
Patri is dative, of Pater. As it is in the dative case, Patri contains the preposition with the Father. So “consubstantial with the Father” is really a perfect translation of “consubtantialem Patri.”