June 23, 2003
A: FROM MENTOR JOAN LUSK IN RI

I think you may mean one unintuitive effect of the theory of relativity. If one twin stays on earth and the other makes a long trip through space at very fast "relativisitic" speed, the one who travel will have aged less than the one who stayed home. Time has run more slowly for her. But from her point of view it looks as if time ran more slowly for the twin who stayed home! Try this explanation:

http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_intro.html
or this one, which has an animation of the twin's trip
http://www.phys.unsw.edu.au/~jw/twin.html

But that might not be what you meant. You might be asking why people get upset at the idea of cloning people when we accept identical twins already. It's not so hard to come up with arguments - both pro and con. Here's a discussion that's more "pro" than I would be - it seems to me that natural twins or triplets are a far cry from deliberately creating many multiple copies of a person.

http://reason.com/9705/col.bailey.shtml