Why are twins not acceptable in the community of The Giver?

Although everyone in the community is the same, and
sameness is a high priority, the people are not EXACTLY the same.  There are no
identical twins.  If a set of identifical twins is born, they are weighed and whichever
twin weighs less is released.  We first learn about the release of identical twins in
chapter 14, when Jonas's father says he has to release one of a set of twin boys once
they are born.

When Jonas requests to view the release in
chapter 19, he comments on why there can't be identical twins.  The Giver says he wishes
they would not release one, and Jonas replies:

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"Well, they can't have two identical people
around!  Think how confusing it would be!" Jonas
chuckled.

Confusion and
discomfort are to be avoided at all costs.  Lily’s story in chapter 14 is an example of
the community’s reaction to identical twins.