When breaking a multi-block block (such as a bed or a door), the block drops two items when destroyed, but only for one of the two blocks.
Beds drop two items if the head part of the bed is broken.
Doors drop two items if the lower part of the door is broken.
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@unknown, I cannot reproduce that. Please file a new ticket and include precise reproduction steps.
I get this bug. Here is how I do it. It has happened like 6 times. It gets dark while I'm out exploring. I find a wall of blocks, dig one block wide, two tall and five deep. Then I go to the very back of the wall, always turn to me left and dig in the wall by one. So now there is a entrance blocks in the wall that is 1 wide 2 tall and 2 deep and the area I'm currently in which is 2 wide, 2 tall, 3 deep...this is enough to put my shulker box down and open it up and get my bed out of it. I go to the entrance to board it up and basically lock myself inside. Then turn around and against the wall in the back I lay down my bed. So now I have my shulker box and my bed down. If I'm at the entrance of the little hole looking at the bed, the box is lighted up with the end of the bed to the right of it. Then I get in the bed to sleep. Wake up at the head of the bed next to the bed and shulker box. Turn around and with my axe or hand, break the bed by the head of it where I woke up. It gives me two beds. Just did it to make sure that I have my description down. If you are at the entrance of the hole here is what it looks like inside from top down.
o o o o o o
o o x b b o
o x x s p o
o o o o o o
in this layout, o = whatever block is there. x = open space. b = bed laid out with head on the right side. s = shulker box. p = open space but where the player spawns when waking up.
also, this, if I lay a bed down, go to the head of the bed and face it and break it with my hand at the head of the bed, i get two beds. ftr, this map I have was started before the pre-release came out and I am on the most recent pre-release 4. I'm playing survival. I just started a whole new game and tried it. it works still. It seems you can break it with any device as long as you do it at the head of the bed. I have done it from all sides. Breaking it at the foot doesn't work. i have used a axe and hand. Still does it. This is a brand new game on the new pre-release 4
Breaking the bottom halves of double block flowers also duplicate it.

I have the same problem with beds. Other blocks I have not tried.
It happens in 16 pre 4, I discovered it and wanted to check if it's been reported.

Maybe this bug also relates to MC-160562. It also drops an extra item, where it shouldn't have when replacing or moving multi block blocks with commands or structures.
Wow, fixed in pre5. Pre-releases get released very fast.
Also happened to me when placed next to my shulker box's no shifting needed
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I want to add something. It also works when a villager interacted with it!
https://bugs.mojang.com/secure/AddComment!default.jspa?id=336927