Here's, how you can duplicate doors in MC:Pe! :D
Take your one door (don't make an more!)
Place door on snow, and out pops 2!
It does delete the block above it though
Take our plank and fill hole :) (or do it out in he open, that should work too)
Leave a comment if you want to see a guide on anything minecraft, and also check out my YouTube channel- www.youtube.com/sircookthebook, I post minecraft stuff!