III. The repetition of the Holy Eucharist on the great festivals is not undesirable.
Again, that our usage may coincide at all points, we wish this thing also to be observed, viz. that when any of the greater festivals has brought together a larger congregation than usual, and too great a crowd of the faithful has assembled for one church to hold them all at once, there should be no hesitation about repeating the oblation of the sacrifice: lest, if those only are admitted to this service who come first, those who flock in afterwards, should seem to be rejected: for it is fully in accordance with piety and reason, that as often as a fresh congregation has filled the church where service is going on, the sacrifice should be offered as a matter of course.
Whereas a certain portion of the people must be deprived of their worship, if the custom of only one celebration be kept, and only those who come early in the day can offer the sacrifice. We admonish you, therefore, beloved, earnestly and affectionately that your carefulness also should not neglect what has become a part of our own usage on the pattern of our fathers' tradition, so that in all things we may agree together in our beliefs and in our performances. Consequently, we have given this letter to our son Possidonius, a presbyter, on his return, that he may bear it to you, brother; he has so often taken part in our ceremonials and ordinations, and has been sent to us so many times that he knows quite well what Apostolic authority we possess in all things. Dated 21 June (? 445).
Source: Letters (New Advent)