Worn out wheel bearing on my nissan Micra 2011?

I have a worn out wheel bearing on my nissan Micra 2011, its basically the wheel bearing hub unit. The machanic said I should replace both or else its will fail again. Is this even true?
Update: Basically the rear right wheel bearing is faulty and he wants to replace both rear right and rear left wheel bearings,
Update 2: basically each wheel at the back takes a wheel bearing hub, you cant replace the bearings only, its a full unit, and they want to replace it on both wheels. sorry my details was a bit rushed
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