At the end when the Night Fox is explaining that he actually stole the Golden Egg on the first night it arrived, there's a flashback montage of how he got through the randomized lasers. He stole the egg, which turns out to be replica #1 that Ocean's team planted. Then a few days later, Ocean's team swipe replica #2 and are caught afterwards.
The Night Fox wanted Ocean's team to think they stole the egg successfully and beat him (so he could lord it over them), so he would have put replica #2 in place of the one he stole. There was no hubbub about it being stolen, the police would have been notified if it was noticed, so the Night Fox definitely placed a replica.
Only, in his laser-dancing scene you can see he is definitely not carrying a replica to swap the "real" one with.
So how in the world did the 2nd fake egg get there?
A rough timeline of the portions dealing with the egg
Second Thursday. Six days left:
Danny and Rusty meet with Le Marc who tells them to put on a show for the Night Fox, and how to beat him. A portion of Ocean's team goes and steal's the Night Fox's paintings.
Second Friday. Five days left:
The other portion of Ocean's team gets on the Paris-Rome train and steals the Egg while in transit (swapping identical bags). Danny immediately goes to the Night Fox, taunts him about the paintings, and pretends to reluctantly accept the Egg challenge, knowing he already has it in his possession. Ocean's team do some recon work at the Egg museum, purely for show (because Le Marc said the Night Fox would probably be monitoring them).
Second Saturday. Four days left:
A portion Ocean's team do more recon work that night, while the Night Fox dances his way into the museum and steals the replica Egg.
Second Sunday. Three days left:
The gang do their final rundown of the plan, and Nagel arrives with his holographic Egg replica.
Second Monday. Two days left:
The day of the fake heist in the museum. All of Danny’s gang gets arrested before they enter the door (as planned) except for a few members. They regroup and try to come up with a new plan, calling Tess and conning her into getting on a plane to Rome.
Second Tuesday. One day left:
Tess and Saul arrive in Rome to complete the lineup for the second attempt at the Egg heist. They make the switch and place a holograph egg on the pedestal, taking the replica, but leading detective is on to them and notices, arresting them all and taking back the bag the egg is in. Linus' mom pretends to be FBI and gets them all out of custody. Rusty takes the real the Egg to Le Marc, while Tess and Danny visit the Night Fox for the big reveal.