Question: When running a web application project, at seemingly random times a page may fail with a CS0433 error: type exists in multiple DLL's. The DLL's are all generated DLL's residing in the "Temporary ASP.NET Files" directory.

Answer: 
I had a partial class with the same name in two different projects.
I solved it by only leaving it in one project.
If the DLL's are showing in a temporary folder, you should try cleaning your solution.