Question 71: What is the ruling on someone who remembers – or doubts – after performing Wudoo’ (ablution) that he has forgotten one of its obligatory acts?
Answer: All praise be to Allaah Alone, and peace and blessings be upon His Messenger, and his family and Companions. Anyone who knows for certain that they have forgotten to wash one of the parts of the body necessarily washed in Wudoo’, which remained untouched by water, must perform Wudoo’ again; while if they doubt after finishing Wudoo’ whether or not they washed one of the required parts, their doubt does not count.
Question 71: What is the ruling on someone who performs Wudoo’ by washing each of the obligatory parts of the body only once so as to catch up with the congregational prayer?
If someone washes the obligatory parts of Wudoo’ only once, their Wudoo’ is valid. There are many authentic Hadeeth that support this...read more here.