12. The five digits $1,1,2,2,3$ can form $\qquad$ four-digit numbers.
12. 30 Consider the exponential generating function $G e(t)=\left(1+t+\frac{t^{2}}{2!}\right)\left(1+t+\frac{t^{2}}{2!}\right)(1+t)=1+3 \frac{t}{1!}+8 \frac{t^{2}}{2!}+\cdots$ $+30 \frac{t^{4}}{4!}+\cdots, \frac{t^{4}}{4!}$ the coefficient of is 30.