The Auric Twin phenomena is usually referring to a type of spirit body attachment that is acting as a parasitic twin to the being, and may need to be removed or cleared to remove energy parasitism.
This can be the result of mismatched body parts, that can be an organic or inorganic event. Sometimes this a result of Soul Fragmentation, Negative Form, Artifacts or from types of alien or dark manipulation like Black Magic. For whatever the reasons, it is generally beneficial to clear this attachment from draining energy from the etheric body double of the person.
A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed by the twin. The occurrence of this phenomenon is sometimes referred to as twin embolisation syndrome or vanishing twin syndrome (VTS), since the 1980s when twin pregnancies were made visible early on by means of ultrasound. Occasionally, rather than being completely reabsorbed, the dead fetus will be compressed by its growing twin to a flattened, parchment-like state known as fetus papyraceus. If the fetus is absorbed completely, there are usually no further complications to the pregnancy. The vanished twin can die owing to a poorly implanted placenta, a developmental anomaly that may cause major organs to fail or to be missing completely, or there may be a chromosome abnormality incompatible with life. Frequently the twin is a blighted ovum, one that never developed beyond the very earliest stages of embryogenesis. In session work we have discovered that this situation can manifest spiritual body complications and may need to be energetically cleared from the Lightbody or ancestral line. 
Term first found: Page 84, HGS Manual