Transfer from Select Box to Other Select Box

[adamthenewbie]

I am trying to alter this script

http://www.quirksmode.org/js/transfer.html

But am having lots of problems as I am not all that great with javascript.

What I am trying to do is:
1) When you move an option from the left side to the right side the option in the left side stays (Now it gets removed).
2) When you remove an option from the right side it gets removed but doesnt get added to the left side.
3) Make sure the same option is not inserted twice into the right side.

Anyway I hope someone can help
Adam