The bug

When you shift-click banners out of the banner input slot of a loom while you already have an incomplete stack of that type of banner in your inventory, it will both stack with that incomplete stack and create another separate ghost item.

Video

The video shows the bug. Even though the video was filmed on a machine which doesn't meet the minimum requirements (which is why the loom background is dark), I have confirmed this bug to also be present on newer machines and other operating systems.

How to reproduce

1. Have multiple banners of the same type (so they can stack) in your inventory.
2. Open a loom.
3. Put a part (not all) of the banners into the loom, so that there is at least one incomplete stack in your inventory, which can accept an amount equal to or lower than the amount of banners in the loom to become a complete stack. (In other words, if you have N number of banners in the loom, the stack in you inventory should be 16 - N or smaller).
4. Shift click the banners out of the loom back into your inventory.
5. The banners from the loom will now have combined with the existing stack, and created a ghost stack.