and his answer was that actually every engine out there basically rendering same geometry 2 times
This makes me glad I'm not using 3Impact. That is completely untrue - most video cards can render anywhere from 2 - 32 texture layers in a single pass. This means that you're not duplicating polygons if your engine supports this feature (like Blitz3D, Ogre, etc.) In fact, I can't believe that 3Impact doesn't support this - it's insane.
so i asked him that and he said, NO, becouse they using vertex buffer so there is no performance impact of that kind (slow down becouse double poly count)
Again, he is grossly incorrect.
The vertex/index buffers store your mesh data on the video card (usually) so you don't have to upload the data every frame (which would be ridiculous - nobody does this anymore). Saying that an engine uses hardware buffers is almost like saying "Hey! My game engine doesn't have a Delay(500) in the main loop!"
Vertex buffer or no vertex buffer, drawing something twice takes 2x the time (minus state changes) - there's no way around that.
"whoever for sure knows whats going on behind scene"
Well, according to the Blitz3D docs:
A little note about multitexturing and slowdown. Graphics cards support a maximum amount of textures per object, which can be used with very little, if any, slowdown. For most cards this is two, but for a GeForce3 it is four. However, once you use more than this amount, Blitz will emulate the effect itself by duplicating objects and textures. Obviously, this may then cause slowdown.
Fortunately, it appears that Blitz3D doesn't duplicate polygons all the time - only when the capabilities of your video card are exceeded (which makes sense).
I honestly can't figure why 3Impact always "emulates" multitexturing when practically every video card out there is capable of multitexturing at the hardware level.