Joined: Dec. 2002
It's fundamentally different from saying a zygote is a unique individual because it is saying that a zygote is one or more, or fewer, individuals.
The "Life Begins at Conception" claim is that some core ("unique") personhood adheres to the ovum at the point of fusion with a sperm.
However, the reality is that one-half of a person, or two people, or quite frequently none, actually result from the event.
Fertilization is therefore not necessarily the event that uniquely determines the identity of a new person. Rather, it's just another waypoint on the road to developing personhood.