'Class FIFinderSyncExtensionHost is implemented in both ...' warning in Xcode since update to macOS High Sierra

There's nothing you can do about this. It's an Apple problem, but it's probably harmless.

Note: I'm not sure what other description could be given than what the warning says. It just means that the same class is defined in two modules. With Apple stuff, they're often identical so it doesn't matter which copy is used.