Why steal the egg twice in Oceans Twelve?

There are two reasons to proceed with conning Tolour. The first is that Tolour might decide he doesn't have to hold up his end of the bargain if they don't actually go through with the "doubled" heist from the museum. That is, his idea is that they need to compete on the same theft. If the team were to steal the egg in a different way, it might not count. They really do need that money.

The more important reason is to take Tolour down a peg. Linus and Danny each mention that he "broke rule number one", which presumably is "don't rat out another thief". The team believes that Tolour needs to have his nose rubbed in the dirt a little bit to teach him a lesson about what happens when you do that.

If you let someone self-inflate first, it's more spectacular when you shoot them down. Tolour was so pleased with himself that he'd beaten them; he couldn't wait to tell them the tale. And he was absolutely crushed when he realized he was wrong.

The Ocean's team didn't particularly want to go to jail or get bumped by Benedict's goons, but it was also very important to them that the "honor" of their profession be preserved.

This is actually one of the basics of confidence tricks, known as The Convincer.

This is the part where the victim (or mark) believes that they have the upper hand, and are convinced that they are going to win whatever it is that's at stake (in this case the stealing of the egg).

The Ocean's gang have to publicly attempt to steal the egg, so that Tolour is of the belief that they have failed to steal it, and therefore he has succeeded.

If they had never attempted to steal the egg from the museum, he would know that something is wrong, because the only reason that they would have to not attempt to steal it is because they already had it.

So it was important to make Tolour believe he had won, in order to humiliate him and teach him a lesson, rather than just outright beating him.

The second theft has nothing to do with beating him, as they had already done that long before he even attempted to break into the museum himself, but had everything to do with conning him.

This is the real reason they had to steal the egg "twice":

If you recall in a black & white flashback, when Danny and Rusty are holding a secret meeting with Gaspar LeMarc, LeMarc says,

"you will have to assume that the moment this challenge begins, Toulour will have you under constant surveillance. You will have to put on an elaborate show..."

Later in the movie they show the second part of the meeting where LeMarc tells them where the real egg is. So, in order to fake out Toulour, they had to "act" out one plan to steal the Egg from the museum, and hold that "act" all the way through until the very end.

If Toulour caught on that they had a different plan and knew where the real egg was, he could've found a way to beat them to the real egg or steal it from Ocean's 11 after they got it. So they had to be 100% convincing in their act all the way through the end stealing the fake egg. This is proven by Toulour and Danny meeting at the end of the movie at Toulour's house where Toulour believes he has won, Ocean's 11 failed, and the "whole crew got pinched." This is further proven by Toulour saying that Tess looks nothing like Julia Roberts, and their plan was never going to work.

Toulour clearly knew every step of their "fake" plan from surveilling them, which they acted out all the way until the very end. Toulour finally realizes he was conned, duped, and that Danny had to have talked to LeMarc to have the real egg, and he lost. This is proven again during an exchange between Rusty and Isabel, where Rusty says Toulour is the best, but he's going to lose. She seems confused. How can you win if you can't beat him and he's better. Rusty knows, it's because of leMarc telling them where the real egg is and Toulour will be stealing a fake. Danny and Rusty admit throughout the movie Toulour can't be beat. So they knew, in NO WAY they could give away that they knew where the real egg was. They had to act out the second heist of the egg to ensure they had the real egg safe.

Why did LeMarc tell Danny & Rusty about the real egg, and not Toulour, his protégé? Because Rusty had one more piece of leverage, LeMarc's daughter, Isabel Lahiri (Catherine Zeta Jones).. Isabel's mother (LeMarc's wife), made Lemarc swear to leave and stay out of Isabel's life forever, or she would turn him in. So he never got to see his daughter since she was a child.

Rusty dated Isabel, and now has met with LeMarc, and Rusty knew that if he could promise LeMarc he could unite him and his daughter Isabel, Le Marc would help them with the challenge and they would win for sure. Lemarc says, "and i'm still getting the better of you two" to Rusty and Danny, because he's getting the real egg at no expense or risk of his own, AND his daughter back, all for just telling them where the real egg is.

There it is!

You need to understand the premise of the film... It starts with Terry Benedict hunting them all down to get his money back (nearly $200m) or he will kill them all.

Now they don't have that money, some have lost it all, some have lost a little. So they cannot get the money back to pay him back. Therefore they have to come up with a plan to pay him back. Toulour has plenty of cash, and only steals for fun, so they hatch a plan to get him to pay it for them and that is why they go through the entire thing. They steal the egg without his knowledge and let him think he's better by stealing it himself etc.

I think there's plenty of holes in the entire plan, but it's fun nonetheless.