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I have dust on my camera too and is annoying af, sometimes i take pictures of stuff i want to sell and people see them as cracks or defects... If its a replacement, you could turn that one in too, iphones 6 are really hard to open...
 
Contact Apple and ask for another replacement, it's nearly impossible to clean the dust there by yourself.
 
if its still under warranty, you can send it back. but the quickest way is to go to a phone shop and it takes less than 5 mins to open that section and clean it. cost very little amount for the service.
 
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