PJ B Hancock and C Foster investigate the double face illusion, where the eyes and mouth are duplicated. On an identification task for briefly (80ms) presented faces, there are strong individual differences: doubling has little effect for the majority, but inhibits recognition for about a quarter of participants. A second experiment shows that some participants are unable to detect face doubling at this speed, while others are 100% correct at 50 ms.
Unlike the Thatcher illusion, a doubled face is still obvious when inverted, but it is less unsettling to look at and a third study found that participants were about 35 ms faster to decide that a face has been doubled when it is inverted. A final experiment tested visual search for normal and doubled faces; neither pops out from the other and the search time per item is again about 35 ms longer for double faces.
The Daily Omnivore 