Can two different electrons superimpose or be at the same place? We know that two different waves can be at the same place, and that electrons show both wave and particle nature, so can two electrons superimpose or be at the same place?
 A: Well, usually you cannot simply add waves, as there is an interaction between the two electrons.
Electrons are identical particles. You cannot distinguish between electron 1 and electron 2. Say, electron 1 sits at x1 and electron 2 sits at x2. Placing electron 1 at x2 and simultaneosly placing electron 2 at x1 should not change anything, because you still have one electron at x1 and another one at x2. This can be expressed as $\Psi\left(x_1,x_2\right)=\pm\Psi\left(x_2,x_1\right)$ (the $\pm$ come from the fact that we are only interested in the probabilities, and not in the probability amplitude). The $\pm$ also implies that the wave-function is either symmetric or anti-symmetric. 
Electrons are fermions, it follows from quantum field theory (but afaik not from quantum mechanics) that fermions must have an anti-symmetric wave function. For the wave function of two particles to by anti-symmetric the two components must differ in any anti-symmetric way. In case of the electrons this is their spin. So YES, electrons can be at the same position, but their spin must differ!
