What are the differences between M.Sc, M.Tech and M.Eng.?
There are some differences, especially in the concept of the programs M.Eng./M.Tech. and M.Sc (and M.A. obviously too).

From my experience, and Wikipedia states this also, the M.Eng./M.Tech. degrees are rather given by Universities of Applied Sciences (at least in Germany, Canada, Finland etc,) and seldomly by Universities focusing on research. Any program that is research oriented will thus very likely give M.Sc. degrees.
The M.Eng. is even given as an undergraduate Master in the UK (see the Wikipedia Link) that gets granted immediately (without a bachelors in between).

For the career of a graduate, the title itself is not too important as aeismail mentioned, since any employer will have a look at what's inside (= the student track record).

Because there are so many slightly different programs, especially in computer/information engineering/technology/sciences I am not sure where universities draw the line regarding the "not granting a 2nd masters degree in the same field/area/discipline" policy.