What necessitates repeating the prayer, and will the first [prayer] be accepted if the worshipper makes a mistake in the the repetition?

A prayer is not to be repeated twice in one day. Repeating it without a reason that necessitates its repetition is among the things forbidden by the Prophet (peace be upon him). However, if one repeats it because it was invalidated, then this is what is obligatory upon him. Its repetition becomes obligatory until it is performed correctly, for an invalid prayer is not rectified by another invalid prayer; rather, it is rectified by a valid prayer.