twinning is completely irrelevant to that fact.
Prior to conception we had two reproductive cells, one from the mother, one from the father and each had 23 chromosomes. When they combine at fertilization they have the same 46 chromosomes that human being will have for the rest of his or her life.
If twinning should occur something very similar to cloning has happened. One or more cells break off and grow into a new separate human being. If someone took one or more of your cells and made a clone of you would that mean you were never a human being up until that point? If it was still a possibility... and it may very well be, does that mean that you are not a human being right now?
"Consider the parallel case of division of a flatworm. Parts of a flatworm have the potential to become a whole flatworm when isolated from the present whole of which they are a part. Yet no one would suggest that prior to the division of a flatworm to produce two whole flatworms, the original flatworm was not a unitary individual.” (1)
Endnote:
Embryo: A Defense of Human Life
Professor Robert P. George, an expert on bio-ethics at Princeton University
p. 148
https://www.amazon.com/Embryo-A-Defense-Human-Life/dp/0385522827/ref=sr_1_1?ie=UTF8&qid=1359831472&sr=8-1&keywords=Embryo+%2Brobert+george