This question has been on my mind for a while. Wizards can multiply food. It is implied by Ron that Molly has used similar charm.
Possibly, but with drawbacks.
The Doubling charm does indeed replicate various objects and I suppose that theoretically you might be able to use the materials just as used to make food. However, there are possibly a few drawbacks to this.
In the following chapter, Hermoine uses the Doubling charm to create a copy of the locket to fool Umbridge.
‘I know that, Harry, but if she wakes up and the locket’s gone – I need to duplicate it ... Geminio! There ... that should fool her ...’
Harry Potter and the Deathly Hallows, Chapter 13, The Muggle-Born Registration Commission
The passage implies that Hermoine's duplicate was "good enough" to fool Umbridge - i.e. unable to create a fully identical copy. With magical objects like potion ingredients, it is entirely possible that "good enough" may be dangerous or even fatal.
Q: It seems that the wizards and witches at Hogwarts are able to conjure up many things, such as food for the feasts, chairs and sleeping bags. . .if this is so, why does the wizarding world need money ? What are the limitations on the material objects you can conjure up ? It seems unnecessary that the Weasleys would be in such need of money...
A: Very good question (well done, Jan!!). There is legislation about what you can conjure and what you can't. Something that you conjure out of thin air will not last. This is a rule I set down for myself early on. I love these logical questions!
Thus we can possibly infer that 1) Doubled ingredients (and potions) vanish and must be used quickly and that 2) this process is potentially dangerous or even fatal.