This will happen within Visual Studio with Web Application Projects (the new project type from Microsoft..... ) if you have nested web sites (e.g. you have a top-level site that has one or more child sites). What happens is the project for the top-level site sees multiple assemblyinfo.cs files (its own and the child sites') when it tries to build, and so fails.
Simply set the build action for the child sites' assemblyinfo.cs files to “None” (within properties), and all will be fine. This is a project level setting, so the project for the child site will compile the asssemblyinfo.cs file normally.