Unfortunately, I guessed the truth about Lars before I saw the reveal; the third time they flashed back to Fry #2 I thought "Wait, is this leading somewhere? Hold on, Lars has a Billy West voice too! Oh no, I figured out the twist!"
Right after that was the wedding scene, though, which had a little intentional reveal where Lars is shocked to find out time clones are doomed. Maybe they wanted me to figure it out early? I'm still disappointed it wasn't covered up better. If it had, it would have been perfect.
They didn't need to use Billy West for both characters; if his voice had been smoke-damaged into a completely different voice, it still would have been believable. And I wouldn't have spoilt myself.
Another thing I'm sore about: this movie ruins "Jurassic Bark." Fry WAS there for the dog, or his time clone was, but the dog wouldn't know the difference. He lived right above Panucci's; the dog would have seen him every day. Now it's not so sad. Bah!