Since journey mode is not meant to be full on creative, it might be a good idea to have a cost associated to duplication, here is how I would implement that
every item has a new attribute consisting of its value relative to others, when researching, any amount of item can be added increasing the amount of available duplication currency and when duplicating the corresponding value is subtracted from it.
you still need to research the item first, possibly sacrificing that amount of value, but overall what this does is that it gives a good sink for useless items and a cap to how much you can duplicate at any given stage, it might also be implemented as an option.
every item has a new attribute consisting of its value relative to others, when researching, any amount of item can be added increasing the amount of available duplication currency and when duplicating the corresponding value is subtracted from it.
you still need to research the item first, possibly sacrificing that amount of value, but overall what this does is that it gives a good sink for useless items and a cap to how much you can duplicate at any given stage, it might also be implemented as an option.