Full Idea
Origin essentialists claim that parental union results in a person, and that person could not have resulted from any other union. However, if the fertilised egg undergoes twinning, at least one of the resultant persons is not the original person.
Gist of Idea
In twinning, one person has the same origin as another person
Source
Trenton Merricks (Propositions [2015], 5.V)
Book Reference
Merricks,Trenton: 'Propositions' [OUP 2015], p.176
A Reaction
Merricks says that therefore that origin could have just produced the second twin, rather than the original person. This is interesting, but doesn't seem to threaten the necessity of origin thesis. Once I'm here, I have that origin, despite my twin.