[Breaking] Cloning a double chest produces two single (empty) chests

Verification builds: 1.6.0.0 - RC0, Beta 15 (980905)

Summary: Cloning a double chest produces two single (empty) chests.

Impact: A double chest clone products two single empty chests.  In the case of Dr Splice's Lab Escape (DSLE); this causes the player to have no gear.  This happens in DSLE in quite a number of areas, so it isn't isolated one particular use-case.

For in-game examples:-

• You can see a Double Chest in DSLE at -123 90 -158.   The chests are cloned via a Command Block (/clone -123 90 -158 -123 90 -159 -123 82 -159 replace). 
• There are 7 double chests at -146 96 -217.  They are cloned into the area directly below.   The clone commands are executed from Command Blocks at -150 99 -204 and -152 99 -204.  Example: /clone -142 96 -207 -149 96 -217 -149 87 -217 replace force.
• There is also another room which fails to clone the chests at -111 86 -219 (filled chests are directly above, as are the command blocks).

Repro Steps:

1. Have a double chest with resources inside.
2. Clone it

Observed Results: Two single (empty) chests appear.

Expected Results: A double chest should appear with resources inside.

Screenshots/Videos attached: https://i.imgur.com/S96f0Nr.png