A mechanism for $\mathrm{A}+2\mathrm{B}\to \mathrm{D}$ is: step 1 (slow) $\mathrm{A}+\mathrm{B}\to \mathrm{C}$; step 2 (fast) $\mathrm{C}+\mathrm{B}\to \mathrm{D}$, where C is an intermediate. The rate law is

The rate law follows the slow step: rate $=k[\mathrm{A}][\mathrm{B}]$. The fast second step comes after the rate-determining step and does not appear; the overall stoichiometric coefficients ($1$ and $2$) are not the orders here.