How can I upcycle old duvets?

Depending on the weight, you can make shower curtains that fit from ceiling to floor. I did this with a cotton duvet and have received so many compliments because I hung the rod at ceiling height rather than the usual shower stall height, then cut an extra plastic shower liner in half (width-wise) to add to the bottom of the current liner so that I now have an inner liner and the decorative duvet on the outside.

See the pillows on this bed? I made the pillows themselves out of down I got from an old duvet. I also made the pillow shams and upholstered the headboard and footboard (I did not make the bedspread) You could remake the duvet. I am assuming you are speaking of duvets themselves, not duvet covers?

Check with your local Humane Society....they are looking for bedding and towels for their animals.

That would be a lot of wonderful fabric- I would suggest window treatments or dining chair upholstery. Possibly a Christmas tree skirt.