I want to preface this by mentioning that I haven't played in an actual PFS scenario yet, so this may end up being a moot point that never even comes up. In the case that it does, though, I'm not sure how I'd handle it.
PFS only has a certain number of scenarios that can be played. Certainly, there are more PFS-registered characters than there are scenarios, so different characters will end up completing the same scenario, even when they weren't in the same group. Then, if these two characters meet in another scenario, this could happen.
"Hail, friend! I'm Hero the Mighty Hero, slayer of Evil Wizard X!"
"You slew Evil Wizard X? Ha! It was I, Adventurer the Great, who slew him!"
Aside from the obvious hyperbole, how should this situation be handled? Both of these characters may have actually killed the wizard, of which (for the sake of this illustration) there is only one.