While the LEUPOLD VX-5HD and VX-6HD have same glass I believe the Duplex in the 5HD models is a much wider Duplex than that in the 6HD models and I think they may be a bit thicker inner plex lines on the 6HD as well.
I have heard this before too. Maybe it depends on what models and reticle. I compared one of my 3-15x44 side by side to the 3-18x44 6HD and it is virtually identical.
Both standard duplex firedot