candidate: Gene duplication , a process which can result in free mutation Chromosomal duplication , which can cause Bloom and Rett syndrome Polyploidy , a phenomenon also known as " ancient genome duplication " Enteric duplication cysts , certain portions of the gastrointenstinal tract Diprosopus , a form of cojoined twins also known as " craniofacial duplication " Diphallia , a medical condition also known as " penile duplication "