Why are you alone in portal

Warning: Spoilers! Do not read unless you want to spoil your portal gaming experience or have played portal.

You are not alone, there were a few test subjects before you, and you, yourself, are a clone (Clues are when GlaDoS says: Your brain is permanently saved and backed up. Also in a secret area in a bonus map, there are loads of chells in test tubes.). But you are alone whilst doing the test because you cant have two people together if you want to make it a fair test, shes testing how good humanity is, blah blah blah, so if its two people together, it would be torture not science.