This may have been answered before back in 2015 sometime, but nevertheless:
Heavy Marker Drone
Attach to an enemy army unit
Double damage dealt to attached unit
Let's say one of those fiendish Heldrakes gets a HMD; now it is dealt three damage by Shi Or'es, an attack from a non-flying unit. The Heldrake's flying keyword reduces the damage to two and then the HMD increases the damage to four. So far so good.
What happens if the Heldrake has two HMDs stalking it? Does it make any difference to the total damage assigned if it had?
I'm a little unsure whether having two HMDs would have any additional effect, particularly regarding timing, since the HMD's text is a declaration/constant effect.