You can come across ‘ambiguous in the namespace’ error sometime in Visual Studio 2008. Error may look like following
GeneratedCodeAttribute' is ambiguous in the namespace .. 'System.CodeDom.Compiler'. … My Project\Resources.Designer
The Real cause is unknown to me, but have faced this challenge and after some research found out and that somehow the some reference path have changed and pointing to GAC. e.g. they were pointing now such as
C:\Windows\assembly\GAC_32\System.Web\2.0.0.0__b03f5f7f11d50a3a\System.Web.dll
Solution: Go in the Project Properties and see if any of the references are pointing to GAC folder, remove each of them one by one and add it again by making sure that the new reference is now pointing to C:\Windows\Microsoft.NET\Framework path.