The episode you are referring to is titled "The Replicator," which is Season 8, Episode 24 of Criminal Minds. In this episode, the team is faced with a serial killer who seems to be copying the methods of past killers. One of those past cases includes a case that was originally investigated by David Rossi.
Rossi, who has a deep personal connection to the original case, has to confront the fact that the son of the original unsub has taken up the same pattern of killing, making the case feel hauntingly familiar. The episode dives into the themes of legacy and how the past can influence the present.
If you have any other questions about the show or need more information, feel free to ask!