When tying a leashed mob to a fence while holding another, a second fence is placed in the same block of the first. By doing so you can bug 2 fences in the same block. When breaking the fence, 2 fences will drop instead. The two fences must be of the same material. Works with any kind of fence.
See video for better explanation.

After more tests it can be assumed that the bug can be replicated infinite times. Please follow the numbered images (video would be too heavy to attach).