On a recent Giant Bomb Podcast a fellow from Valve said they hadn't heard of the theory that Chell in the Portal games is a clone. This was a common explanation for the
drawings behind walls
in Portal 1 and 2. It also provides a tidy in-universe explanation for respawning. I thought there was some reference to clones made by GlaDOS in the first game. What evidence, in both Portal games, can be found that indicates that the test subjects are cloned Chells?