The sign of Gemini has a connection with the good and evil twin. Dr. Jekyll and Mr. Hyde and the strange case of the split personality. All double sign personalities are centered upon the light and dark aspects of the personality/dual experiences/two natures, the other double-bodied signs are Sagittarius and Pisces. The duality of human nature with both good and evil existing in us all is the main point of stories like Jekyll and Hyde. And if we can’t accept our darker side it is often projected outside, or we try to hide our repressed side, it’s the part of us we don‘t want others to see.
The twins represent the stark opposite nature of dark and light. Sibling rivalry comes from many of the twin myths with one brother attempting to kill/repress the other. However, both need to be brought into consciousness and somehow integrated into the personality. Gemini hates to be trapped by the weight of his own mortality. The duality and dark/mortal side of Gemini have to do with the fact that this is a double sign. The shadow side pertains to all human beings, and we all have our own dark/repressed natures. The hidden self has often been locked away but desperately wants consciousness.