The "XamlCTask" task failed unexpectedly in Xamarin
<p>I am currently having a problem building my Xamarin.Forms solution. I'm getting an error of <code>The "XamlCTask" task failed unexpectedly</code>. I tried looking at the Xamarin logs and Stacktrace but I can't seem to find what's the issue. Any suggestions where I can start looking?</p>

<p><strong>Logs</strong></p>

<pre><code>&lt;E2ETraceEvent xmlns="http://schemas.microsoft.com/2004/06/E2ETraceEvent"&gt;
&lt;System xmlns="http://schemas.microsoft.com/2004/06/windows/eventlog/system"&gt;
&lt;EventID&gt;0&lt;/EventID&gt;
&lt;Type&gt;3&lt;/Type&gt;
&lt;SubType Name="Warning"&gt;0&lt;/SubType&gt;
&lt;Level&gt;4&lt;/Level&gt;
&lt;TimeCreated SystemTime="2016-10-11T10:45:07.4999078Z" /&gt;
&lt;Source Name="Xamarin.VisualStudio.Android.Designer.MonoAndroidDesignerService" /&gt;
&lt;Correlation ActivityID="{00000000-0000-0000-0000-000000000000}" /&gt;
&lt;Execution ProcessName="devenv" ProcessID="19260" ThreadID="1" /&gt;
&lt;Channel /&gt;
&lt;Computer&gt;XXXXXX&lt;/Computer&gt;
&lt;/System&gt;
&lt;ApplicationData&gt;An unexpected error occurred trying to initialize Android Designer.&lt;/ApplicationData&gt;
&lt;/E2ETraceEvent&gt;
</code></pre>

<p><strong>Stack Trace</strong></p>

<blockquote>
  <p>Severity  Code    Description Project File    Line    Suppression State
  Error     The "XamlCTask" task failed unexpectedly.
  System.ArgumentException: An item with the same key has already been
  added.</p>
  
  <p>Server stack trace:     at
  System.ThrowHelper.ThrowArgumentException(ExceptionResource resource) 
  at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue
  value, Boolean add)    at
  Xamarin.Forms.Xaml.XamlParser.ParseXamlElementFor(IElementNode node,
  XmlReader reader)    at
  Xamarin.Forms.Xaml.XamlParser.ReadNode(XmlReader reader, Boolean
  nested)    at
  Xamarin.Forms.Xaml.XamlParser.ParseXamlElementFor(IElementNode node,
  XmlReader reader)    at
  Xamarin.Forms.Xaml.XamlParser.ReadNode(XmlReader reader, Boolean
  nested)    at
  Xamarin.Forms.Xaml.XamlParser.ParseXamlElementFor(IElementNode node,
  XmlReader reader)    at
  Xamarin.Forms.Build.Tasks.XamlCTask.ParseXaml(Stream stream,
  TypeReference typeReference)    at
  Xamarin.Forms.Build.Tasks.XamlCTask.Compile()    at
  Xamarin.Forms.Build.Tasks.XamlCTask.Execute()    at
  System.Runtime.Remoting.Messaging.StackBuilderSink._PrivateProcessMessage(IntPtr
  md, Object[] args, Object server, Object[]&amp; outArgs)    at
  System.Runtime.Remoting.Messaging.StackBuilderSink.SyncProcessMessage(IMessage
  msg)</p>
  
  <p>Exception rethrown at [0]:     at
  System.Runtime.Remoting.Proxies.RealProxy.HandleReturnMessage(IMessage
  reqMsg, IMessage retMsg)    at
  System.Runtime.Remoting.Proxies.RealProxy.PrivateInvoke(MessageData&amp;
  msgData, Int32 type)    at Microsoft.Build.Framework.ITask.Execute()<br>
  at
  Microsoft.Build.BackEnd.TaskExecutionHost.Microsoft.Build.BackEnd.ITaskExecutionHost.Execute()
  at
  Microsoft.Build.BackEnd.TaskBuilder.d__26.MoveNext() XXXXXXX.XXXXXX.XXXXXX</p>
</blockquote>