Will a fully collapsed extension ladder safely support twice its duty rating?
Your logic is that in fully-collapsed mode, both channels are doubled up, and ought to be able to bear twice the weight.  

The problem with that is the second channel is not resting on the ground.  It is still transferring all its borne weight to the first channel at some point.  

What's more, it is virtually impossible to distribute your weight evenly on the rungs 100% of the time.  You will be climbing or descending on toe or heel.  You might be super careful the first few times, but complacency  will set in.  

They do make heavy-capacity ladders... but I am concerned about the general wisdom of taking a great deal of mass up a ladder.  Agility is a big factor in ladder work, and I can't imagine being able to be safe managing 120 pounds of gear up a ladder, even if it was securely attached to me, even if I had more muscle than I do.