Steps to reproduce:

1. Open the backpack
2. Hold Shift & press the left mouse button to take the item and esc to close the backpack at the same time.
3. It's all

Video: https://youtu.be/BvOvosIYRQQ

Hallo,

and thanks for reporting this.
I guessed that this was in, we will take a look asap ...

Cheers
~MrTroble

Thanks for reporting!
I could recrate the dupe bug with this little macro script:

Robot bot = new Robot();
bot.keyPress(KeyEvent.VK_SHIFT);
bot.mousePress(InputEvent.BUTTON1_DOWN_MASK);
bot.keyPress(KeyEvent.VK_ESCAPE);
		
bot.keyRelease(KeyEvent.VK_SHIFT);
bot.mouseRelease(InputEvent.BUTTON1_DOWN_MASK);
bot.keyRelease(KeyEvent.VK_ESCAPE);

This happend only on 1.12.2 (tested 1.14.4)

Fixed the dupe bug, at least I cannot reproduce it anymore 💃
Also added backpacks in offhand are now useable