Hello, Understanding your problem that is you are trying to back up your previous iPhone to an iPhone 8 which is earlier backed up on your MacBook Pro. This is not a big issue usually people are facing this issue when they are updating ios to 12. This issue is seen you just need to contact to the technical support team by dialing the mac support number. The tech support team is just a call away to solve your issue so what are you waiting for dial now and get your issue reolved in minutes.
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