Titration Question

[rinner]

Hey, can anyone help me with this question?
4 Oxalic acid (C2H2O4) is a diprotic acid. The concentration of an oxalic acid
solution was determined by titration with a standard sodium hydroxide
solution, using phenolphthalein indicator. A 20.00 mL aliquot of oxalic acid
was titrated with a 1.25 M solution of sodium hydroxide. Titres of 21.06,
21.00, 21.08 and 22.06 mL were required to reach the end point.
Given that the end point occurred when both acidic protons on each oxalic
acid molecule had reacted, calculate the oxalic acid solution’s:
a molar concentration
b %m/m concentration.
I keep getting 0.822M instead of 0.658M, and I'm not sure what I'm doing in this question.
Thanks in advance

[sweetcheeks]

Can you post your working out? It makes it a lot easier to see where you potentially have gone wrong.