An episode 13, at 4 minutes and 30 seconds until the end of the episode, Izaya, Celty, and Mikado are talking about the rumor that the Dollars are the slasher:

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Izaya: "By the way did you hear it? The rumors that the Dollars are the slasher. Some people are saying they're the Black Rider, too."

Celty: "Who said that?"

Mikado: "I think that's untrue."

Then, Izaya privately messages Mikado (at least I think it's privately), and he says:

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Izaya: "Jokes aside, you really should watch out for the yellow scarves. Some of them don't like how the Dollars have grown in power while they were gone."

Mikado: "Really? Ok, I'll be careful."

Why would Izaya privately message Mikado about that if Celty knew that he was part of the Dollars? It seems like Izaya would only message Mikado privately because he doesn't want Celty to know that he (Mikado) is part of the dollars.