Can hydrazine be protonated twice? First, $\ce{N2H4}$ is a base when it reacts with water, it accepts a proton.

$$\ce{N2H4 + H2O -&gt; N2H5+ + OH-}$$

Can I follow up the reaction with this?

$$\ce{N2H5+ + H2O -&gt; N2H6^{2+} + OH-}$$
Yes, it is possible, but only to a very small amount.

The $\mathrm pK_\mathrm a$ of the hydrazinium (2+) ion in water is $-1$ (CRC Handbook of Chemistry and Physics, Ed. D.R. Lide, CRC Press, Boca Raton, FL, 2005), so the equilibrium

$$\ce{N2H5+ + H2O &lt;&lt;=&gt; N2H6^2+ + OH-}$$

is almost completely on the left side.