This is the problem of running duct work in the attic. Did you use R8 flexible duct? If you are going to add more insulation then it would be easier use a type which has no vapor barrier. It is common to install ducts in an attic and then cover them with blown in insulation.

I used R6. Read later the difference to R6 is marginal. I live in Florida so it is very humid. I have read both ways with and without a vapor barrier could cause condensation issues. Without a vapor barrier the lining of the flex duct will be colder than the air temperature because of the new insulation and might cause condensation. My understanding is that you also shouldn’t have two vapor barrier‘s.

So I am a bit confused but want to do something to reduce the heat gain.

i

One of my thoughts is to take off the existing foil vapor barrier and wrap new insulation over it with foil barrier