If I'm right, you want the player to drop an item into a hopper linked to a chest, and this chest would contain the item twice?
Well it's quite simple : the following command will do the job. However, you will need to trigger it once the item gets into the chest (something like comparator), and take care to make it only triggerable once so the player could not exploit it.
Some requirements : the player must only drop 1 item (not a stack) and the chest must be empty in order to work
/data modify block [Chest XYZ] Items.Count set value 2
How it works : The command search for the number of items in the first chest's filled slot, then set it to 2. Notice that even non-stackable items can be stacked this way. However, if the item has not reached the chest yet when the command triggers, nothing will change.
Rollback Post to RevisionRollBack
If you know there is a contest for winning a minecraft account, I'm interrested!
Otherwise, if you are interrested to buy an account for me in exchange of command/datapack making, I'll be happy to work for you!