Occasionally while breaking blocks i will get two of the same block and they are both valid and not ghost blocks of any kind. This is especially apparent when taking down a beacon pyramid or repeatedly placing and breaking a shulker box. For example when building a beacon i will put down two stacks and 36 blocks and once i break them all i will get two stacks and 42 instead. Below is a link to a gif of a shulker box duplicating.

Shulker Box Duplication

In addition, this may be caused by the fact that my tools use the unbreakable tag. Also this behavior has shown itself with and without the effects of haste.