This problem usually happens as a result of the shadow assembly mechanism of the Visual Studio IDE. Fixing it involves closing all of the Visual Studio IDE instances and loading your project again.
Let me share some more information:
To avoid locks, the Visual Studio IDE makes "shadow" copies of the assemblies which are actually used by the application and loads the assembly from the shadow copy location instead. This way it is possible for a developer to delete an assembly or replace it safely. Unfortunately, when replacing an assembly, the IDE makes a second shadow copy, which it loads again. Thus, there are two assemblies having the same namespaces, classes, etc. loaded in the current Application Domain and an ambiguity occurs. When viewing a page in design mode, the definitions of the classes in these two loaded assemblies are in conflict, hence the error.
Can you try restarting the solution?
the Telerik team