If you get hit with the following .NET Entity Framework (EF) error, the solution might be very easy and (seemingly) not related to the error message details.
Column names in each table must be unique. Column name 'MyColumn' in table 'MyTable' is specified more than once.
If you receive this message and are certain that the column is not duplicated (not sure how this would happen anyway), you might want to check your connection strings. I created a new console app to do some quick testing and forgot to add my connection strings to the App.config file. I ran the code (that worked in another project) and got the above error. I noticed that the server associated with the error was
server: (localdb)\mssqllocaldb
This is when I realized my mistake. All it took was to add the connection strings into the new App.config.