What is the ruling on one who believes that performing ablution (wudu') once is not sufficient, even though he knows that the the Prophet (peace and blessings be upon him) performed ablution once?

The Prophet, peace and blessings be upon him, performed ablution (wudu') once, twice, and thrice. Washing a limb twice or thrice is recommended, not obligatory. What is obligatory is to thoroughly cover the limb with water. Scholars have agreed that performing ablution by washing each limb once is sufficient, twice is better, and thrice is best. Scholars have disliked extravagance in it and exceeding the action of the Prophet, peace and blessings be upon him. If what is meant is that a single wash is not sufficient without thoroughly covering the limb with water, then this is correct.