I've been doing a lot of heavy layout design in v12 over the last few weeks. While there are many issues with the new set up, I have yet to unintentionally duplicate a layout object by clicking or double clicking it.
Duplicating an object requires more than a mouse click, the control (windows) or option (mac) key must also be down at the time of the mouse click. Since these modifier keys aren't used in layout mode except for duplicating an object with a drag of the object, (And yes, it doesn't work right in 12 as has been previously reported), I'm puzzled as to how just clicking or dragging an object is also duplicating it for you.
i have seen weird similar situations where a defective keybord sent CTRL or Maj or etc... permanently to the computer. Anyways, i suggest to test it with another keyboard.
To make the unintentional duplication happen, in layout mode, hold Shift Key and select a series of fields and/or labels. If your hand holding the mouse is not steady and clicks on a field with a little dragging action, duplication will happen.
Sorry, but I cannot reproduce this behavior with the shift key down. The shift key helps manage mutliple selections by selecting deselecting individual items without affecting the "selection status" of other layout items so I woudn't expect to see this result in a duplicated layout object.
If I were holding down the CONTROL key, I would expect that as this is the modifier key for the "duplicate while dragging" action.
I will note that these two keys are close to each other and will ditto Fred's suggestion that your keyboard may have an issue here.