Then how do you explain the, ahem, "alien jizz" all over the keyboard if HE was the one who made it. The aliens wouldn't have touched it if he had already created it.
If he was really a freshly-minted clone, he may have already had it on himself without realizing it. If the stuff "evaporates" away quickly, and the keyboard was the first thing he really looked at closely, he may have inadvertently smeared it there himself.
Or, being a freshly-minted clone, maybe his brain was still wiring itself up and receiving the massive dump of memories needed for him to think he was Sean Buckett, and he hallucinated it. Would explain how sticky green goop just .. disappeared. Seems more likely than the other explanations, in my opinion.