I am running armored 12/2 bx to wire 4 high hats, the armored cable is grounded. There is a heating duct which I must go over, but the AC will come in contact with it... Is this ok or do I need to avoid having it make contact?
There is no such thing as BX cable. Are you working with Type AC cable (armored cable), or Type MC cable (metal-clad cable)?
Metal-Clad Cable (Type MC)
Type MC cable has a grounding conductor, and the sheath cannot be used as a grounding conductor.
If you're working with MC cable, it's not likely any fault condition could lead to the ducting becoming energized.
Armored Cable (Type AC)
Type AC cable has an internal bonding strap, which is used as a grounding conductor.
If you're working with AC cable, there are indeed fault conditions where a duct in contact with the armor could become energized. I'd try to avoid contact between the sheath and metal objects. In either case, make sure the grounding strap is properly bonded.
NOTE: The fault conditions required to energize the ducting would be, improper bonding, and a ground fault (current flowing on the sheathing).
Apart from electrical considerations, one should avoid having dissimilar metals in long-term contact, especially in what could be damp or humid conditions. Electrolytic corrosion could compromise one or the other of the metals. That's why, for example, you need to use copper clamps and copper nails to fasten copper water pipe...
When properly installed the cladding of both AC and MC cables are always grounded (in most cases using identical connectors) and both cable types have always been installed in contact with most of the usual conductive building materials: steel studs, structural steel, ducts, plumbing and so on. And because Armored Cable has the bonding strip, its sheathing would have the lower resistance of the two cable types making it the safer one if its sheathing had to conduct a fault current.
Because many more people know exactly what you mean when you say Romex, I prefer the word. When it comes to the terms BX , AC or Armored Cable, it depends on who I'm talking to and it also depends on whether it's the old steel stuff or the contemporary aluminum. For the old stuff, because more people would know what I mean, I'd be more likely to say BX and reserve the terms AC or Armored Cable for only when referring to the newer stuff and talking to someone who knows what it is.
Why Armored Cable? Is this your home? Is it elsewhere wired in Romex? Why #12 awg? Is it a 20 amp circuit? What kind of Armored Cable do you have? Is the sheathing aluminum? Is there a full size bonding strip? Is there a green insulated copper ground conductor?