The midseason finale for "Arrow" Season 5 teased the return of Laurel Lance (Katie Cassidy) when she appeared inside the Arrow bunker at the very end of the episode. However, a brand new extended trailer reveals that she is not the Laurel fans have come to know throughout the show's run.
Oliver aka Green Arrow (Stephen Amell) is shown welcoming Laurel back home, believing she had been resurrected somehow. However, in a separate scene Laurel is shown using a sonic scream. When she does this Oliver immediately states "That's not our Laurel."
Laurel upon hearing his comment turns to him and states it was about time someone had figured out the truth. This would further indicate that she is not the same Laurel of Earth-1 and is instead a different Laurel from a different reality.
This version of Laurel is actually from Earth-2, where she is a meta human supervillain known as Black Siren, according to Screen Rant. She was first introduced in "The Flash" Season 2 where she revealed she could create a sonic blast just using her voice. This is different to Earth-1 Laurel who could only do this with the aid of gadgets.
By the end of "The Flash" Season 2, Black Siren was defeated and imprisoned in STAR Labs. Given that she appeared inside the Arrow bunker it is surmised that she may have escaped or that the "Flashpoint" event from "The Flash" Season 3 could have altered events where she never got caught in the first place.
This would rule out previous theories about how Earth-1 Laurel may have been brought back to life. The Lazarus Pit was ruled out quickly since it was destroyed and the only possible resolution was that the "Flashpoint" event altered the timeline, allowing Laurel to live, according to Cinema Blend.
"Arrow" Season 5 will return with episode 10 titled "Who Are You" on Jan. 25, 2017. The extended trailer for the episode can be watched below: