How to insulate HVAC Ducts in maintenance when the insulation get damaged ?

First of all assess the Situation on the extent of the damage. Then remove the damaged insulation+plaster (wrapping material) to show the duct work inside. Refill the new insulation (Foam, Rockwool etc) pack the insulation in place and apply the plaster and allow to cure.

It depends on the damage it self,, if you are using fiberglass insulation and the damage was only on the jacketing layer you can cover it by the approved alluminum duct tape that you're using to tight insulation..But if the damage was severe, the old insulation should be removed and clean the duct surface from the old adhesive , put adhesive again and insulate as normal but never put new insulatiion on old adhesive. 

almnium duct tap in case small damage or remove the clean the reinstall