Ran into this one last night while playing, doing The Nuka Cola Challenge (MS05).
Sierra Petrovita's two speech checks, the base one asking her what the reward is, and the Medicine one asking her about Quantum's side effects are both repeatable, and continue to award XP. The base speech one also comes in her dialogue as [Charisma 25], although it is correctly checking Speech and is passable.
Looking in FNVEdit, I think it's entries 49C44 MS05TLMoreAboutSoda03Medicine and 38B81 MS05TLMoreAboutReward, although I'm not super familiar with dialogue trees so I can't see the exact fault.
No other mods are affecting the quest, only Fallout3.esm, TTW.esm and TTW_SpeechChecks.esp.
