Why am I having an ambiguous type error when calling a WebMethod on ASP.NET Web Application?-Collection of common programming errors

The message sent to browser is as follows:

The type My.API.Class is ambiguous: it could come from assembly ‘[on Temporary ASP.NET Files]’ or from assembly ‘[on bin folder]’

The problem occurs when debugging a Web App, specifically when making a request to a WebMethod of a WebService.

The project compiles just right. It generates My.Website.dll on bin folder and if I publish the Web Application. It works fine.

The asmx file is on the root of the application. The CodeBehind file is on App_Code and its marked to be compiled to generate My.Website.dll.

I should be missing something really important.

I found someone having the same issue with a possible related cause. Check it out. The way this person exposes its problem is somehow similar but I get starting to be lost when he talks about a proxy class and shared dlls I don’t use.

Any help is appreciated.