System.ArgumentException: Duplicate type name within an assembly

Question

I am working on an ASP.Net MVC 3 web application using EF 4.1. Since today, I am getting this error:

System.ArgumentException: Duplicate type name within an assembly

Answer 1

I ran into this error today, too. It turned out, that my data model has missing the foreign and primary key constraints in the data base. I had copied the original database to a local database to speed up the execution, but I had only copied the data and not the relationships. Other errors i had in this context were

an item with the same key has already been added

and

Unable to update the EntitySet Table because it has a DefiningQuery and no InsertFunction element exists in the ModificationFunctionMapping element to support the current operation

it all was due to a faultive database structure