A galaxy 9 bLY away. A supernova went off there in 1995. Astronomers observed it. Then they observed the same indecent again in the sky 20 years later in 2005. That's because of gravitational lensing. Light taking different paths to reach us. Some 20 years earlier some later.
This supernova was observed even before 1995, and possibly will be again later.
.
NYT coverage.
https://www.nytimes.com/2015/03/06/science/astronomers-observe-supernova-and-find-theyre-watching-reruns.html
.
Excellent visual coverage by Derek.
https://www.youtube.com/watch?v=ljoeOLuX6Z4
.