I think, if there is only a 0.1 bar pressure difference with a single square meter door, around the weight of a ton would be needed to open it - if it is sealed hermetically and opens outward.
Is it possible?
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Firstly: Flightdeck doors are to the best of my knowledge not hermetically sealed.
Secondly: Doors will normally have one of several solutions employed to safely handle explosive decompression:
Needless to say, you need one solution in either direction. Whatever pressure difference there is it will most probably very quickly be equalised and there should not be any difficulty in opening the door if need be.