edge split/smoothing group performance on game engine ?

Everyone knew that edge split in Blender is equivalent to smoothing group in Max (they create different smooth surfaces on the same mesh). The difference is edge split creates extra vertices (actually it splits the mesh along the hard edges and leaves lots of duplicate vertices), so this maybe bad for game models where minimum vertex count is required.
The above is what I’m supposed to know, but I don’t really know how a game engine works, do duplicate vertices affect the performance badly ?

It really only apply if You subdivide the mesh. For game models You more likely to model in the poly-count You want to use in the game to get the most out of the polys You use…

You could use edge split, apply the vertices, and then remove the duplicate vertices (Remove Doubles), but that may destroy the edge splitting effect - I’m not sure. Duplicate vertices don’t hurt the performance, but a lot of faces hurt performance.

The 3D engine creates separate vertices for each face anyway.
So there is no need to worry about the douplicated vertices, as long as you do not produce douplicate faces.