My bad. I had never heard the expression “once and again” and would be puzzled if I heard it. But apparently it’s a legit phrase defined fairly well online. Over and over again just suggests multiple repetitions. The enemy bombed the city over and over again, I.e. many times. The mother said to the child, “I’ve told you over and over again to take your dirty shoes off before coming in the house”
@Gordo99 It is ok. It has happened to me as well that there are questions about vocabulary or expressions in my mother tongue which I'd never come across before. Thanks for the insight on your region's dialect.