BC30560: 'ExtensionAttribute' is ambiguous in the namespace 'System.Runtime.CompilerServices'.

Can anyone please help me try and track down this error (I've moved from VWD 2008 > VS 2012)
Message: BC30560: 'ExtensionAttribute' is ambiguous in the namespace 'System.Runtime.CompilerServices'. Source Error:
[No relevant source lines] Source File: InternalXmlHelper.vb    Line: 9
<div class="expandable">Show Detailed Compiler Output:</div> <div id="compilerOutputDiv"> C:\Program Files (x86)\Common Files\Microsoft Shared\DevServer\11.0> "C:\Windows\Microsoft.NET\Framework\v4.0.30319\vbc.exe" /t:library /utf8output /R:"C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Web.ApplicationServices\v4.0_4.0.0.0__31bf3856ad364e35\System.Web.ApplicationServices.dll" /R:"C ...