Titration homework question

My friend sent me this for his homework, and I haven't done a titration calculation in years.



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> A student titrated $\pu{10.00 mL}$ aliquots of her unknown amino acid solution with standard $\pu{0.1521 M}$ $\ce{NaOH}$ and with $\pu{0.0986 M}$ $\ce{HCl}$. No inflection points were observed with the $\ce{HCl}$ titrations but two were observed in the titration with $\ce{NaOH}$. One occured after titrant had been added and another after a total of $\ce{18.61 mL}$ of $\ce{NaOH}$ had been added. Calculate the amino acid concentration. HINT: Assume the $\pu{10 mL}$ unknown solution contains a mixture of the $\ce{A}$ and $\ce{A+}$ forms of amino acid.
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So based on the information, I have a diprotic amino acid. I know at the second equivalence point, mol $\ce{H2A}$ = mol $\ce{A^2-}$, so if I can find mol $\ce{A^2-}$, I'm basically done.


At the second equivalence point I have added 


$$\pu{0.1521 M}\, \ce{NaOH} \times \pu{18.61 mL}\, \ce{NaOH} = \pu{2.83 mmol}\, \ce{NaOH}$$


If mol $\ce{A^2-}$ = mol $\ce{NaOH}$, then my concentration of $\ce{H2A}$ would be: $\pu{2.83 mmol/10 mL = .283 M}$


I don't think that's right because I haven't even used the first equivalent point for anything, and I don't think it's necessarily true that at the 2nd equivalent point, mol $\ce{A^2-}$ = mol $\ce{NaOH}$, so I'm not sure how to proceed.


