does anybody knows what kind of mirror should i use to avoid double vision? tnx in advance....

The obvious and safe answer is a front surface mirror. That way, you simply don't have an extra surface that may cause undesired "bonus" reflections or other problems.

Now, whether or not these extra reflections in a normal glass back surface mirror are actually visible under normal cenditions is another matter. The amount of light reflected from a glass surface depends on the angle, so it might turn out that it's insignificant if you keep the angles sensible.

I think I saw a comment on this from someone around here who has actually tried non-FS mirrors, but I'm not sure...

I'd use FS mirrors anyway, just in case. If nothing else, it avoids losing lumens due to passing through the extra surface and glass twice (in and out) for the reflection, and I'm not very keen on experimenting with bordercase stuff just to (potentially) save a few $ anyway.

I sell the FS mirrors. Write to sales@lumenlab.com if you need one.

tnx for the reply guys. i'll try to find one here near in my place and just incase they dont have it then maybe i have no choice but to buy from lumenlab...