When using the setblock command to generate a random chest, I put 2 together to make a double chest, then I faced the double chest and created another, then to the right, another, two double chests facing each other and it made a chest with a bigger capacity and as far as I can tell it duplicated items. Protection needs to be set in so commands cannot put three chests next to each other.

The command used (also in the screenshot):
/setblock ~ ~ ~ chest 6 replace