You have:
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
The second
xmlns="http://schemas.microsoft.com/winfx/2006/xaml"
is a duplicate of the first. You can only have ONE root XML namespace which is the "xmlns" without any postfix. The offending line duplicates the next one which has the proper postfix (notice the :x)
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"