Question: Is it a coincidence that Needy uses the same knife that killed Jennifer to kill the band? Needy wouldn't have know about the knife since she wasn't present when Jennifer was killed.
Answer: Maybe coincidence but I think she might knew its the same knife the bad used to kill Jennifer because of the power she got from the demon after being bitten by Jennifer aka the succubus.
Answer: When Jen was explaining how the band members through the knife in the waterfall. When needy was walking and found the knife, the knife was beside one of the balls that the scientists were testing where the water went to. It showed the scene where the scientists were putting little red balls down to see where they came up. So no it wasn't a coincidence, she knew that was the knife.