Repeat that again I believe is what they call to be a pleonasm because ‘‘again’’ is inherent to ‘‘repeat’’.
My observation is that, for some reason, people tend to say ‘‘Repeat that again’’ quite often and I have to admit that I do too, even though I know I shouldn’t.
Anyway, I would say that is true when people mean to ask someone to repeat something that was said just once.
“Repeat that again” is correct wherever it requests further repetition. It is incorrect wherever it requests initial repetition.
If I return a defective item once, I have merely returned it. If I have to return it a second time, I must return it again. There’s no difference between that and “repeat.” Can a person be rehabilitated more one than once? Indeed.