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It seems that you are running the local WACK Test for your app. Based on the error message, the Microsoft.UI.Xaml failed to install because it is installed before. The local WACK test uses your local device to test and it is optional. I'd suggest you upload your package to the store submission directly. When you upload your package to the store, there will be another Online WACK test for your app and the store will only check the result of the online WACK test.
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