Is it permissible for my aunt, who was ill and vowed to pray with each obligatory prayer the same number of rak'ahs as that obligatory prayer, to do so?

The obligatory prayer is discharged by a person performing it once in the prescribed manner. If a significant defect occurs that affects the validity of the prayer, it must be repeated. However, if the defect does not affect the validity of the prayer, repetition is permissible to seek a more complete reward. Repetition is also permissible to attain a virtue, such as praying with tayammum (definition) and then finding water, or praying alone and then finding a congregation. If there is no reason for repetition, then repetition is not prescribed, based on the Prophet's (peace be upon him) saying: "Do not pray a prayer twice in one day." One who does so should intend it as an absolute supererogatory prayer or a Sunnah (definition), because an obligatory prayer is not repeated except for a reason. A vow is not binding except through verbal articulation.