SUPER IMPORTANT! There is a bug that only happens in the plastic chests which allows you to duplicate any items you put in the chest. However you have to be playing with at least 1 other person.
This is how it works. Imagine that I (Aqua Water) am with a friend (John).
I open the plastic chest and put a plank in it.
I shut the chest.
John opens the chest after I put the plank in and takes out the plank.
What is meant happen here is that John has the plank and I don't but instead,
I go to open the chest, the plank is still in there for me .
I pick up the plank and then we both have the plank.
This can be repeated by
I open the chest, shut the chest.
John takes out the plank.
The plank is still there for me to take, but if I don't take it;
I can open the chest, shut the chest..
John takes out the plank, and repeated.
This works for any item in the game.
I Hope this helps!