RNSlider Windows has a built authoring issue on how it consumes RNW

[adamgorMSFT]

<removed>\node_modules\react-native-windows\PropertySheets\External\Microsoft.ReactNative.Uwp.CppLib.targets(12,5): warning MSB4011: "<removed>\node_modules\react-native-windows\Microsoft.ReactNative.Cxx\Microsoft.ReactNative.Cxx.vcxitems" cannot be imported again. It was already imported at "<removed>\node_modules\@react-native-community\slider\windows\SliderWindows\SliderWindows.vcxproj (80,5)". This is most likely a build authoring error. This subsequent import will be ignored.

The way RNW is being consumed from SliderWindows needs to be fixed/updated. The proper way is to consume via the RNW targets and props now.

I will likely open a PR to address this soon.

[tero-paananen]

#618