Moments before Nathan closes Arthur's eyes the "dead" Arthur Petrelli clearly blinks.
Ando's power-enhancement ability takes the form of red lightning identical to that which Hiro sees Ando apparently shoot him with in the future (The Second Coming - 3x01), possibly adding a new interpretation to those events.
Two items shown on the "Previously on Heroes" segment were not actually shown on previous episodes--Mohinder telling Arthur Petrelli to put his hands on the container of the formula, and Ando and Parkman showing a comic book to the messenger dispatcher.
When Daphne gets zapped into the past, she does not see an exact repeat of the scene. The dialog is slightly different, and the stutters and pauses indicate that it was a different recording of the lines.
President: What exactly are you asking for?
Nathan: I want to open up a facility where they can never hurt anyone.
President: OK Senator. Lets make sure you get what you need.
Nathan: Thank you... Mr. President.
Matt: Daphne! Daphne! Where'd you just go?
Daphne: I don't know. I was out there, but I was watching myself in here. And you were there. (points to Ando) And you were there. (points to Matt) And... Matt, I think I traveled back in time.
Sylar: You are not my mother. Are you? And I can tell if you're lying.
Angela: No I'm not.
Sylar: For a brief moment I wished you were.
Ando: I have powers now! I rescued you!
Hiro: I was about to tear up the formula! Couldn't you have come later?
Ando: Remind me to never save your life.
Sylar: That's the thing about the truth, Claire. Stings like a bitch.
Mohinder: (speaking into a dictaphone) I've run out of time. The infection is spread to my lungs. By giving myself abilities, I've signed my own death certificate. My hope is that by injecting myself with the formula, it will be the cure.
Sylar: You're not hunting me, Noah. I'm hunting you.
Sylar: Arthur Petrelli is dead. No need to go to Pinehearst now.
Sylar: Don't worry Claire. Peter couldn't do it so I did. Just like Mommy wanted. I know I repulse you. Terrify you. You see me as a monster. And yet, you did this to me. And before the night is over, I'm going to prove to you, one by one, that you're all monsters. Exactly like me.
Claire: I'll hunt you down and kill you.
Sylar: From cheerleader to stone cold killer? Who's the monster now?
Claire: (shoots the intercom) You are.
Mohinder: There is good and there is evil. Right and wrong. Heroes and villains. And if we're blessed with wisdom, then there are glimpses between the cracks of each where light streams through. We wait in silence for these times when sense can be made, when meaningless existence comes into focus, and our purpose presents itself. And if we have the strength to be honest, then what we find there, staring back at us, is our own reflection, bearing witness to the duality of life. And each one of us is capable of both the dark and the light, of good and evil, of either, of all. And destiny, while marching ever in our direction, can be rerouted by the choices we make. By the love we hold onto, and the promises we keep.
Nathan: Why did you save me? Why did you do it?
Peter: Because you're my brother, and I love you.
Nathan: It's not what I would have done.
Knox: I'm going to wait until your brother has gotten rid of all the formula.
Nathan: You're working for Peter?
Knox: Yeah. He's one of the good guys now.
Sylar: On the sixth day, God created man in his own image. Now it's up to us to figure it all out. Right, wrong. Good, evil. In each of us is the capacity to decide what drives our actions. So what is it that makes some choose selflessness, the need to devote oneself to something greater, while others know only self-interest? Isolating themselves in a world of their own making? Some seek love, even if unrequited, while others are driven by fear and betrayal. There are those who see their choices as dark proof of God's absence, while others follow a path of noble destiny. But in the end, good, evil, right or wrong, what we choose is never what we really need. For that is the real cosmic joke. The real gift that God has left behind.
Hiro: Stop! We have come for the formula.
Daphne: It doesn't belong to you.
Tracy: And you are...
Hiro: Hiro Nakamura. It is my sacred duty to see that it is destroyed.
Tracy: Sorry, I've got plans for this little piece of paper, Pikachu.
Hiro: Pikachu... (bows to Tracy) Excuse me. (punches her in the face, Tracy lands on her butt and he takes the formula from her)
Nathan: Eventually you are going to have to choose a side.
New Zealand: February 18, 2009 on TV3
Latin America: February 20, 2009 on Universal Channel
Belgium: April 23, 2009 on VT4
Germany: November 25, 2009 on RTL II
Finland: May 1, 2010 on Sub
Czech Republic: August 27, 2010 on Prima COOL
Slovakia: August 1, 2012 on Markiza
Ando: Ando gains the ability to temporarily enhance others' powers. This manifests as a blast of red lightning.
This episode was formerly known as War, Duality, and Duel, before getting its final name, Dual.
The wound on the right side of Flint's face is shaped like the symbol.
Sylar does the opening monologue for this episode.
One of the three villains Noah releases is Echo DeMille, the main character of the Going Postal webisodes on the official Heroes site.
Daphne: And you were there, and you were there...
This is a nod to L. Frank Baum's novel The Wizard of Oz and the film based on it, in which Dorothy wakes up and describes who was in her "dream". The fact that Daphne, like the main character in the novel, is from Kansas also relates to the situation.
Daphne: Back in a flash.
This is very possibly an allusion to the DC comic book character the Flash, who has the power to run at super fast speed. Some versions of the character have been able to run faster than light, and even to travel through time, just as Daphne is about to do in this scene.