If the volume of a parallelopiped of sides $\vec{a},\vec{b},\vec{c}$ is given by $$\det\begin{pmatrix}\vec{a} & \vec{b} &\vec{c}\end{pmatrix}$$ But if two of those vectors are equal then $\det\begin{pmatrix}\vec{a} & \vec{b} &\vec{c}\end{pmatrix}=0$.
Here how can the volume of a paralleopiped with two equal sides be 0?
Try drawing it. A vector is an object with a length and direction. If two vectors are equal, then they have equal length and more importantly for this problem, they have the same direction. If you draw two of the three sides on top of one another, you get a parallelogram. A parallelogram has no volume.