"Làm ơn lặp lại!"

Exactly what I thought when I heard it.

Could it also be "xin lặp lại"?

To me:

It sounds like "please do it again!" because the first time wasn't good enough

In which situation can this phrase be used?

I think "Please, repeat again" should also be accepted.

a more natural English would be "please, repeat it", although it is not accepted