What are the results of high energy electron electron collisions? Are other particles created?
As far as I know, nobody has ever done this, at least not at what we currently consider high energy. (Electron-electron collisions happen at low energy all the time, of course.) I doubt that anything interesting would happen, primarily because electrons are mutually repulsive, and they have a low mass. That means two colliding electrons would just bounce away from each other. They would produce some photons from bremsstrahlung radiation, but there wouldn't be any nontrivial interaction.
Even if we did give the electrons high enough energies to get very close and possibly have some sort of interesting interaction - and we're probably talking at least tens or hundreds of TeV here - the possible products are constrained pretty well by conservation of electron lepton number and charge. In particular, the reaction products would have to have total charge -2 and electron lepton number +2. The only configuration of known particles that satisfies that condition is two electrons, and if there were an unknown particle that could change that, it would have to be very strange indeed. Other than that, some number of particle-antiparticle pairs could always be produced, but we get that already at higher energies from $e^+e^-$ and hadron collisions, so there's no particular motivation to build a machine that can produce electron-electron collisions.
Let's try just considering what could happen based on conservation laws. The two electrons have a charge of -2e, so the end product must as well. Lepton number conservation is required also, and we have $L_e=2$ here. At this level, it looks difficult to produce additional particles which satisfy just these two conservation laws. If you work in QED the only vertex is the photon one, and so if the energy of the two incoming electrons is large enough you might be able to produce something like
$$e^- +e^-\rightarrow e^-+e^-+(e^++e^-)$$
You can see the term in parenthesis are lepton-number and charge neutral. In principle you could do this with $\mu^++\mu^-$ or $\tau^++\tau^-$ as well.