One way could be to do the selection in a loop. In the loop, check if a different individual has been chosen. Once this happens break from the loop.

From: "Sahin, Cem" <[log in to unmask]>
To: [log in to unmask]
Sent: Wednesday, November 12, 2008 8:02:29 PM
Subject: Roulette Wheel Selection

Re: A specialized run after the first one
I am using one-point crossover and roulette wheel selection. I want to be sure that I am performing crossover operation between different chromosomes. (source[0] and source[1] are always different).  How can I do it?
Cem Safak SAHIN