ninachen03

Verified this issue with Visual Studio 17.10 Preview 7 (8.0.40 & 8.0.3 ). Can repro it. code as below  

@PureWeen here is the sample project. https://microsoft-my.sharepoint.com/:u:/p/v-dongmc/Eeda192t-rRJsJjFfCZeM5IBCZ_18GVEiqui_g0DQPlgig?e=CmI7oI

Verified this issue with Visual Studio 17.11.0 Preview 1.0 ( 8.0.40 & 8.0.21& 8.0.3 ) Can repro it. Xamarin form cannot repro it. 

I can repro this issue on the latest 17.11.0 Preview 2.1(8.0.60 & 8.0.21) When adding duplicate default style for "Border" or others in Styles.Xaml, No error message is reported during...

I can repro this issue on the latest Visual Studio 17.6.12(build410) (8.0.61 & 8.0.21)

I can't see the code you provided. Could you provide us with a sample project so we can investigate it further? Looking forward to your reply!

This issue has been verified using Visual Studio 17.12.0 Preview 1.0 (8.0.8). Cannot repro on Android platform. 

Thanks for the detailed information, I reproduced your problem on Visual Studio 17.12.0 Preview 2.0(8.0.90 & 8.0.80 &8.0.3) 

This issue has been verified using Visual Studio 17.12.0 Preview 3.0(8.0.92) iOS version is 17.5. Cannot repro it. 

This issue has been verified using Visual Studio 17.12.0 Preview 2.0(8.0.90 & 8.0.3). Can repro this issue at windows platform.