A Polish Stamp. The World Assembly Of Builders Of Peace Warsaw 1977.

I'd appreciate your feedback on my stamp. Is this a unique defect?

The stamp has a doubled blue background (shadow). The faint reflection of the blue graphic with the white dove of peace is lowered by 6mm towards the bottom edge of the stamp.

I've attached photos of the stamp in various lighting conditions. It's worth viewing it from different angles and on a screen larger than your phone. I've attached a demo image to illustrate the extent of the offset.

The effect is better visible to the naked eye than in the photo.

What does the back look like?

This effect is almost invisible from the adhesive-coated back. It's only slightly visible when held up to the light.

Additionally, I don't know if this is important, but there is a clear trace of blue ink on the back.

What could be the cause of this error?

It looks like some cylinder picked up blue ink that has not completely dried from a stamp and at a next cycle deposited it on another stamp, Nice variety, not really an error. To weak to command a large premium.

Certainly not unique, but unlikely to be common for this issue.