Title text: Is he ALSO wondering at what point our thoughts diverged, if they even have yet? 'dude, I think he just took your credit card' AM I THE ORIGINAL? HOW DO I TELL?
The term "identity theft" refers to a thief acquiring various types of a victim's identification (for example, bank account number and/or Social Security number), thus allowing the thief to pretend to be the victim and commonly steal money from the victim's bank account, etc.
This comic exaggerates the term, interpreting it as the thief literally acquiring the victim's whole personality, where the victim here appears to be Randall himself. The thief has apparently taken on Randall's love of space as well as his existential angst. The character Megan also expresses existential angst in 625: Collections. Thus, the thief is now completely overwhelmed by having an entirely new personality, not to mention one whose parts clash with each other.
The title text is spoken by Randall, in which he wonders about the specifics of the identity acquisition process: at what point did their thoughts diverge, if ever? Presumably, even someone whose personality is complete copy of someone else's would still have different thoughts at some point. This is depicted in the comic itself. The thief is baffled by his newly acquired interests, while Randall himself would not be baffled by the thoughts and interests he's developed over his entire life. Randall also wonders whether his new doppelganger is wondering the same thing, which could imply that their thoughts have not fully diverged.
Someone else comments that Randall may be overthinking the situation, and that the thief only took his credit card information (a common method of identity theft), which would not transfer personality traits. However, Randall's anxiety has overwhelmed him. He ignores the speaker and expresses his worry that there is now no way to discern whether he is the original Randall or the "copy."
- [Randall sits in front of a laptop, looking at his hands]
- Randall: I feel paralyzed by overwhelming existential dread.
- Randall: ...and yet for some reason I'm really excited about space?
- The thief who stole my identity has a lot to deal with.