Commentary for Berakhot 42:1
או צבור וצבור אבל יחיד לגבי צבור כמאן דלא צלי דמי קמ"ל ואי אשמעינן הכא משום דלא אתחיל בה אבל התם דאתחיל בה אימא לא צריכא
or to a person who said the Tefillah with the Congregation and is repeating it with the Congregation ; but having said it alone, he is like one who has not prayed when he finds himself with a Congregation ! Therefore he informs us [that having once said the Tefillah in private, there is no need to say it a second time with a Congregation], And if he had only given us the latter teaching, [we might have supposed that he does not say the Tefillah a second time with the Congregation] because he has not actually commenced it; but in the former circumstance, having commenced it, therefore he should not [interrupt it on discovering his error] ! Consequently it was necessary [to have the first teaching of Rab Judah].