Answer
Diprotic acids are capable of donating 2 protons to the solution, and carbonic acid ($H_2CO_3$) is an example.
Work Step by Step
When a diprotic acid is added to a solution, it will happen this reaction:
$H_2A(aq) + H_2O(l) HA^-(aq) + H_3O^+(aq)$
But, that doesn't stop there, because $HA^-$ is capable of donating a proton too, and that makes $H_2A$ as diprotic acid.
$HA^-(aq) + H_2O(l) A^{2-}(aq) + H_3O^+(aq)$
** The second reaction happens in a lesser quantity.