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It's my understanding that after updating to macOS High Sierra, your Mac is now stuck in a startup loop. I may be able to help with this. I would suggest trying to boot into safe mode: Use safe mode to isolate issues with your Mac If you're able to boot into safe mode, restart your machine and see if it gets stuck in the loop again.
If that doesn't help, use this article to reinstall the macOS: How to reinstall macOS
Let us know if this helps.
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