How does in-line Fresnel holography produce twin images?

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hologram graphic — holography Figure 1: Gabor's original method for creating holograms.

In in-line Fresnel holography, the production of twin images is a consequence of the hologram acting as a zone lens.

When a hologram is created, it functions as a summation of zone lenses, with each point on the object contributing its own pattern. Upon coherent illumination, this zone lens structure produces two distinct beams: one that converges to form a real (conjugate) image and another that diverges as if originating from a virtual (primary) focus.1

Because in-line holography uses a collinear background, the light associated with both the real and virtual images propagates in the same direction. Consequently, when viewing one image, the light from the other appears as an out-of-focus, overlapping component. This interference between the two images is a characteristic limitation of the in-line Fresnel technique, which is why the method is generally restricted to objects with significant transparent areas.1